Suppress addCoreDBData() in tests overriding the revision schema.

Suppress addCoreDBData() in tests overriding the revision schema.

Some unit tests for the MCR schema migration change the DB schema
in the test setup. However, addCoreDBData() will not work with the
modified schema. Since these tests don't actually need addCoreDBData()
to do anything, they can simply override it to do nothing.

Without this change, tests for Ic2221da30c and Ifabf39e12ba843
fail with $wgMultiContentRevisionSchemaMigrationStage = MIGRATION_WRITE_BOTH.

Change-Id: I86e140ec981dfa4e904822b1600399c533f9e3d6