[1LP][RFR] Fix AttributeError in pytest_polarion_collect

[mshriver]

The method called in the VM fixture to process the polarion docblocks assumes that the session cache attributes have already been set

A call to set_cache will make sure that they actually are created, avoiding an AttributeError during collection

pytest --collect-only --dummy-appliance -m 'requirement.ansible'

<redacted>

INTERNALERROR>   File "/home/setup/repos/integration_tests/cfme/fixtures/vm.py", line 30, in pytest_collection_finish
INTERNALERROR>     process_json_data(session, session.items)
INTERNALERROR>   File "/home/setup/repos/integration_tests/.cfme_venv/lib64/python3.7/site-packages/pytest_polarion_collect/utils.py", line 351, in process_json_data
INTERNALERROR>     if not (all_items or in_test_dir(str(item.fspath), session._test_dirs_cache)):
INTERNALERROR> AttributeError: 'Session' object has no attribute '_test_dirs_cache'

[jarovo]

I did hit this. Thanks for fix.