Steps to replicate the issue:

• Install and configure an extension that uses virtual domains, e.g. CheckUser.
• Run PHPUnit tests.

What happens?:
Watch in horror as preexisting data from the shared tables used by the extension is wiped out and test database prefixes as well as test data is leaked.

What should have happened instead?:
The tests should either use local tables or manage table prefixes and associated bookkeeping for each virtual domain.