Convert MigrateDrupal6Test into a reusable abstract class which can be reused for D7 and other migrations

Problem/Motivation

MigrateDrupal6Test loads all the dumps of the different Drupal6 migrations, and tests the complete migration.
We will need similar behavior for MigrateDrupal7Test, MigrateMultilingualDrupal6 et al.

Proposed resolution

Create a MigrateFullDrupalTestBase abstract class, and move the tearDown and testDrupal methods, so they can be reused.

Remaining tasks

User interface changes

None.

API changes

None.