[1LP][RFR] Fix marker_filters logic

[tpapaioa]

Previously, the filter logic for the tier, requirement, and customer-scenario options was not working as expected. For example, specifying "--tier 1" did not work unless you also included the --requirement filter with a matching requirement. This PR fixes the logic so that each filter is independently applied. Specifically, the fix is to apply the requirement filter only if one or more requirements were actually specified.

Example, before fix:

$ pytest --collect-only --tier 1 cfme/tests/configure/test_zones.py
[...]
collected 11 items / 11 deselected                                                                                                                                      
[...]

$ pytest --collect-only --tier 1 --requirement Configuration cfme/tests/configure/test_zones.py
[...]
collected 11 items / 10 deselected / 1 selected                                                                                                                         
<Package /home/tpapaioa/git/integration_tests/cfme/tests/configure>
  <Module test_zones.py>
    <Function test_zone_crud>
[...]

After fix:

$ miq-runtest --collect-only --tier 1 cfme/tests/configure/test_zones.py
[...]
collected 11 items / 10 deselected / 1 selected                                                                                                                         
<Package /home/tpapaioa/git/integration_tests/cfme/tests/configure>
  <Module test_zones.py>
    <Function test_zone_crud>
[...]

$ pytest --collect-only --tier 1 --requirement Configuration cfme/tests/configure/test_zones.py
[...]
collected 11 items / 10 deselected / 1 selected                                                                                                                         
<Package /home/tpapaioa/git/integration_tests/cfme/tests/configure>
  <Module test_zones.py>
    <Function test_zone_crud>
[...]

{{ pytest: -v --collect-only --tier 1 cfme/tests/configure/test_zones.py }}

[john-dupuy]

Nice PR!

[mshriver]

Great find @tpapaioa, nice work!