Add tests to include instead of packages

pyproject.toml:
Include the tests using poetry's integration for includes instead of
packages, as the latter leads to the tests being added to packages in
the generated setup.py.

Fixes #11
3 jobs for issues/11 in 5 minutes and 54 seconds (queued for 3 seconds)
Status Name Job ID Coverage
  Test
passed coverage #18246

00:00:48

passed integration #18247

00:05:53

passed linter #18245

00:00:56