Split the self_test part of the tests into another file

[zz1874]

This PR is a fix of self_test (#123) depending on #362.

The self_test always failed when we added the functionality of parsing typeshed packages like types-setuptools because it wasn't installed in our virtual environment of CI.

In our tests.yaml, we have

     - name: Install project
        run: poetry install --no-interaction --sync --with=nox,test

before

      - name: Run fawltydeps on Python ${{ matrix.python-version }}
        run: poetry run nox --no-venv --no-install --non-interactive -s self_test -- -vv

while running fawltydeps against itself needs to install all optional dependencies, including "nox", "test", "lint", "format", "dev", and types-setuptools is a dependency of lint.

We cannot just install the projects with all optional dependencies in tests.yaml because in our unit tests, we checked click package as an example, which will be installed when including format, and then the tests/test_install_deps.py::test_resolve_dependencies_install_deps__via_local_cache will fail because it will use LocalPackageResolver instead of TemporaryPipInstallResolver. (The detailed logs are here)

So, after thinking of this, splitting the self_test part of the tests may be a better solution. In this way, in the self test, we install all dependencies and optional dependencies and do a complete test, and in the normal tests, we do the remaining tests with only "nox" and "test" groups installed.

[Nour-Mws]

Thanks for figuring this up @zz1874! This was not straightforward to debug.
I took a little pause on the inclusion of all deps groups in the nox session as I imagine lint would be enough. But then reasoned that since we're testing FawltyDeps as a project, it makes sense to include all deps groups in pyproject.toml in the environment we're using to resolve the deps.
Could you please reduce this PR to the last commit only? We can rebase PR #362 once that's done and re-run the CI there.