A function was written in `test_fixtures.py` to drop a test database
template if the corresponding database id doesn't belong to a file.
Alongside this fact, every file that is written is removed after 60
minutes. Meaning any potential database template can never exist
longer than one hour.
This follow-up work was added to deal with the potential race
conditions when running `test-backend`. Ensuring that all templates
are properly dealt with.
Essentially rewritten by tabbott for cleanliness.
Fixes the remainder of #12426.
6b033c7909