Several migrate_drupal migrations fatal error on count()

Pretty simple, actually, to add source count checking for all migrations based on MigrateSqlSourceTestCase. Unfortunately, the fake DB driver doesn't actually catch these failures (although it adds a couple unrelated ones) - @phenaproxima's upcoming patch to replace the fake driver with SQLite should address that.

See #2499835: Remove broken Fake DB driver - with phenaproxima's patch, my fail test still doesn't fail, apparently because the particular problematic source plugins are either untested or using MigrateDrupal6TestBase instead of MigrateSqlSourceTestCase, thus evading the failure... I'll need to add/modify those tests.

The current failures reported have nothing to do with the present issue, fwiw - just another demonstration of why #2499835: Remove broken Fake DB driver is necessary.

Fake driver is dead, let's see if the fail test can get the right failures now...

So, the expected behavior was that tests for migrations improperly implementing count() would fail and the rest would succeed. What actually happens is that when I run the tests locally they all pass (e.g., UserTest has both an expected and actual count of 2), but testbot failed on all of them. I understand the problem locally - the migrations I'm trying to catch are not based on MigrateSqlSourceTestCase, where I added testSourceCount(), so they're evading my test, and I need to figure out how to properly test them. I have no clue why testSourceCount() fails on everything under the testbot, however. If I force a failure locally I see a message like "Failed asserting that 3 matches expected '2'.", but unfortunately testbot does not provide that detail, which makes it difficult to see what the problem is.

Anyway, in local testing, a partial (but probably not complete) list of problematic migrations includes:

• d6_user_picture_field_instance
• d6_user_picture_entity_display
• d6_user_picture_entity_form_display
• d6_upload_field_instance
• d6_upload_entity_display
• d6_upload_entity_form_display

OK, this should fail as expected for the broken source plugins. Note that the actual assertions I added aren't getting the chance to fail, because it fatal errors out from under them...

Next - fix it!

OK, here's the patch with the fixes. Note that most of the volume here is simply changing migrate_drupal tests involving a single migration, or a set of migrations with a clear primary migration, to save the migration in the class so the general testSourcePlugin() can test it.

Sure... First, a question I meant to ask earlier - any thoughts on the query itself? What's the simplest query to use just for the sake of returning a query object to fulfill the method's contract, in instances where that query will never actually be executed?

Before I submit the updated patch, what's a good name for the generic method (to be called by any source plugins not needing a real query)? dummyQuery()?

Here we are with the trait suggested by phenaproxima. Note the interdiff is sort-of-not-really-incomplete, the patch in #15 no longer applies to MigrateDrupal6TestBase.php, but there were no changes to the changes (if you know what I mean) in that file.

Here's the reroll, necessitated by #2514168: Streamline migrate_drupal integration tests. I grumbled at all the test changes with merge failures, until I realized that the streamline patch meant I didn't have to change all those tests... interdiff doesn't work because the old patch doesn't apply any more, but this reduces the patch to about 10% its former size.

Finally putting that uniqid() into the dummy query exposed a missing count() method...

Tiny docblock improvement, leaving RTBC with @phenaproxima's blessing.