feat: add tests for custom test runner

Add a test that running `./manage.py test` results in both an exit code
of 2 along with an informative error message.

While adding the test, it was noted that there was some inconsistency in
behaviour since tests share a common directory in which copier output is
generated. This causes some cross-talk between test runs.

Ensure that a fresh temporary directory is used for each run of each
test.

To avoid creating virtualenvs within vitualenvs and repeatedly
installing the same packages over and over again, when poetry is used
directly--as opposed to being used by tox--set the env used to be the
system Python.