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 Job ID Name Coverage
  Test
passed #18246
coverage

00:00:48

passed #18247
integration

00:05:53

passed #18245
linter

00:00:56