This fixes flushing test databases when two aliases point to the same
database.
Use a list() to store the test database aliases so the order remains
stable by following the order of the connections. Also, always use the
"default" database alias as the first alias to accommodate `migrate`.
Previously `migrate` could be executed on a secondary alias which
caused truncating the "default" database.
Backport of 06e5f7ae1639f1e275e7cc1076dc70ca3ebaa946 from master