ci(pytest): Fix coverage due to pytest plugin change

Moving from conftest.py to pytest_plugin.py (which comes via a
setuptools entry point) requires a workaround to prevent loss
of coverage from real libtmux tests relying on our pytest plugin.

See also:
- https://pytest-cov.readthedocs.io/en/latest/plugins.html
- pytest-dev/pytest#935 (comment)

Author: tony

.github/workflows/tests.yml

@@ -68,7 +68,11 @@ jobs:
           export PATH=$HOME/tmux-builds/tmux-${{ matrix.tmux-version }}/bin:$PATH
           ls $HOME/tmux-builds/tmux-${{ matrix.tmux-version }}/bin
           tmux -V
-          poetry run py.test --cov=./ --cov-report=xml
+          poetry run py.test --cov=./ --cov-append --cov-report=xml
+        env:
+          COV_CORE_SOURCE: .
+          COV_CORE_CONFIG: .coveragerc
+          COV_CORE_DATAFILE: .coverage.eager
       - uses: codecov/codecov-action@v2
         with:
           token: ${{ secrets.CODECOV_TOKEN }}