Mark slow tests

Add decorator @pytest.mark.slow that will skip a test by default.

The command-line option --runslow, which will enable these tests, has been added to the CI script.

build-and-test-py-project.sh

@@ -85,6 +85,6 @@ if test -f $REQUIREMENTS_TXT; then
   $PIP install -r $REQUIREMENTS_TXT
 fi
 
-${PY_EXE} -m pytest -rw --durations=10 --tb=native  --junitxml=pytest.xml -rxsw test.py
+${PY_EXE} -m pytest -rw --durations=10 --tb=native --junitxml=pytest.xml -rxsw --runslow test.py
 
 # vim: foldmethod=marker