Write PHPUnit tests for 6.x Comment source plugin

I'm a little confused about how to write a d6 comment source plugin test, since in order to really test it we will need a database to query to see if query goes ok(assuming the new definition of source).
A mock of this piece will end up in a test which does not really test anything.
So, if we include a d6 dump, then we could see if the expected values are returned since we have a value to compare with on the dump, is that the original plan?

I was planning to mock DBTNG. Why that wouldn't test anything?

makes sense, will try to star on this.

A poor initial patch.
It seems not that straightforward to mock dbtng. In the other side IIRC phpunit tests could not access database, which sounds fine since we are using the configured database directly, but I am not sure if I am assuming correctly.

Some questions:

• Where SqlBase::query() method on the source is executed? (I could not find one place)
• What exactly to test, i.e. only methods on MigrateSourceInterface?

Have you had a chance to follow up on this? I have similar questions :)

EDIT: So I managed to get a CacheBackendInterface object by doing the following:

$this->cache = $this->getMock('Drupal\Core\Cache\CacheBackendInterface');

I found this in Drupal\locale\Tests\LocaleTranslationTest.php :)

I see in the repository that this test is running, chx++