Add error msg to assertions in MigrateSourceTestBase

Problem/Motivation

It is too often too difficult to find out which array key has the error in a source plugin test. The expected arrays are very similar (for example, lots of keys with a value of 0 or 1), and the error messages, do not indicate in which array the error is occurring. Sure using the debugger or commenting out arrays will work but it would be a lot simpler if the test reported the array index.

Proposed resolution

Add a informative message to two assertions where this is most needed when looping through all the plugin rows.

Lately I have been using this and it had proven helpful.
sprintf("Value at 'array[%s][%s]' is not correct.", $i - 1, $key);

Remaining tasks

The usual.
Decide on the wording of the error message.