Write tests for rollback

Adding a small change.

The tests are failing because the rollback itself is not clean. Also the following error is also present in the watchdog while doing the rollback manually.

exception: [Warning] Line 232 of core/lib/Drupal/Core/Entity/EntityStorageBase.php:
array_flip(): Can only flip STRING and INTEGER values!

This appears to be a bug with the migrate module itself.

Debugging it further.

This needs to happen asap.

Re-rolled for core and fixing a problem that happens when a migration has failed and is marked as so...

Whilst Drupal\migrate_drupal_ui\Tests\d6\MigrateUpgrade6Test running field_test_phone fails... and then rolling it back without this patch is problematic.

+------------------------------------------------------------------+----------------------------------+---------+----------------------------------+-------------------+-----------------+---------------+------+
| source_ids_hash                                                  | sourceid1                        | destid1 | destid2                          | source_row_status | rollback_action | last_imported | hash |
+------------------------------------------------------------------+----------------------------------+---------+----------------------------------+-------------------+-----------------+---------------+------+
| 083571826d1529ac6c11ada1990b89793c6f1d5d917afcd5952a79e63ce771de | field_test_three                 | node    | field_test_three                 |                 0 |               0 |             0 |      |
| 0bbd2cb3bdd8f42575a24ba2e8a8ac774a70099ac798f47b5cf3ffd4183b148d | field_test_two                   | node    | field_test_two                   |                 0 |               0 |             0 |      |
| 0c92025b1752e9eb2bf33d8fb60e2255c6a397ad5704c462efacc7656986a54c | field_test_exclude_unset         | node    | field_test_exclude_unset         |                 0 |               0 |             0 |      |
| 118aedab42ce4ca69f70d5f330fb46d68cd2dff853f8f0657e59e24be231677f | field_test_link                  | node    | field_test_link                  |                 0 |               0 |             0 |      |
| 2d56cd0c02fdf2d4e6bf646f4242ef610a36eb516e5ec581e19d7226076cce1e | field_test_phone                 | NULL    | NULL                             |                 3 |               0 |             0 |      |
| 3cc00c4d5095800f1027a9f1174c55b74ab8da31bd3db5b11501d64bcd40f5cd | field_test_text_single_checkbox  | node    | field_test_text_single_checkbox  |                 0 |               0 |             0 |      |
| 4039c6e968de126fd7fd9e0c1c285f413a264726d8d1342c89047618d07224bb | field_test_float_single_checkbox | node    | field_test_float_single_checkbox |                 0 |               0 |             0 |      |
| 42a2c22c7b701a57c57ea83686c4c401c1c291fa31fc7d7cad9dd47ea6749648 | field_multivalue                 | node    | field_multivalue                 |                 0 |               0 |             0 |      |
| 6b90cb0c8fa2c3442ff26ba40d368292ac75c945786c4a8214d87bfcc72eca49 | field_test_datestamp             | node    | field_test_datestamp             |                 0 |               0 |             0 |      |
| 6f49fc7bc561b25f173b936a88865235d75487eec2e04b1d0c3f3b04b0eeb28a | field_test_datetime              | node    | field_test_datetime              |                 0 |               0 |             0 |      |
| 735cecdbd545a4314f8e71cc5f1ba9293a249c087535ca4de5874e4761ba93ad | field_test_email                 | node    | field_test_email                 |                 0 |               0 |             0 |      |
| 80da953fccb58a20fe92dbc600bc69d1e5c97425bbd125a7373c435f30d11c39 | field_test_filefield             | node    | field_test_filefield             |                 0 |               0 |             0 |      |
| 892bab3a818b518379c0017fa215325df61e0aa5862f8135655fa5625799330b | field_test_date                  | node    | field_test_date                  |                 0 |               0 |             0 |      |
| 8ac85be3b972469a1d902203d13db2de18338eda9ee4ddd4ea2f9eb93cf3da10 | field_test_text_single_checkbox2 | node    | field_test_text_single_checkbox2 |                 0 |               0 |             0 |      |
| 8b9dd3d89855fbf69149a905f142a5c773a81e097a54428b008102b44e1f411f | field_test_integer_selectlist    | node    | field_test_integer_selectlist    |                 0 |               0 |             0 |      |
| 9a6911936b9b32570e8f6c709a3022676e5aee516d2c034e05c4ac5cb8ba141d | field_test                       | node    | field_test                       |                 0 |               0 |             0 |      |
| aa73636ec55e89cdca4d2d2c5a0c1914a2f459f39707b4b1f007ce22443e88cd | field_test_four                  | node    | field_test_four                  |                 0 |               0 |             0 |      |
| ae44c68d140122b51966d69fa7ccb4a5bfcd46da4eb85891c018fb2f5278a276 | field_test_imagefield            | node    | field_test_imagefield            |                 0 |               0 |             0 |      |
| d042e1e3accdca9b6a22d6db2d99e02401e8bae17418a87314199f0dda053b1e | field_test_decimal_radio_buttons | node    | field_test_decimal_radio_buttons |                 0 |               0 |             0 |      |
| d651a35f521119809933bc8b8eeebcc35d9226284883a568ba0edd2dcbe0a84c | field_test_identical1            | node    | field_test_identical1            |                 0 |               0 |             0 |      |
| e89f7ca0acabecfef820a4643097feda36d0bc4f9ae4ac4eafee0fd195095ce6 | field_test_identical2            | node    | field_test_identical2            |                 0 |               0 |             0 |      |
+------------------------------------------------------------------+----------------------------------+---------+----------------------------------+-------------------+-----------------+---------------+------+

Rollback was removed from the UI, and tests are part of the issue to reinstate them: #2687849: Add back rollbacks on migrate_drupal_ui.