Problem/Motivation
In #3074043: Move simpletest.module DB-related functions to TestDatabase, deprecate we're trying to deprecate top-level simpletest module functions by moving them to Drupal\Core\Test\TestDatabase.
It so happens that we can't test many of our changes, because TestDatabase is mostly static and thus can't be mocked.
Proposed resolution
Figure out how to modify TestDatabase so that it can be tested.
Modify.
Test.