correct path of custom test runner and add tests

Correct the path to the custom test runner set via the TEST_RUNNER setting.

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.

Closes #130 (closed)