N = self.parallel templates are created, and these templates were
previously named 'zulip_test_template_<1, N>'. However, to support
running multiple instances of `test-backend`, a unique
`random_id_range_start` was created for each template database.
There was no problem prior because the templates would simply be
used again and thus did not require any clean up. Now that there are
unique database names being created, every time `test-backend` is run
these templates can accumulate on disk. Instead, we clean up our
templates at the end of every complete run of the test suite, or upon a
SIGINT.
Fixes: #12426