change test selection to flags

[blu-nitro]

Changes test selection to flags, which are easier to understand.

1. --model
   • possible types: nk3, nkpk
   • if nkpk: specific tests are excluded
   • defaults to nk3
2. --test-suite
   • possible types: basic, normal, full, slow
   • selects tests of different slowness factors
   • normal includes basic, full includes normal, slow includes full
   • defaults to basic
3. --virtual
   • if not set: exclude virtual tests
4. --hil
   • if set: exclude tests not to be run on hil (currently ssh)

This removes the necessity of selecting markers or keywords explicitly.

Does not stop users from selecting markers / keywords explicitly: To use them properly the options --test-suite slow --virtual should be used, to not skip any tests by default.

TODO:

• [ ] Decide if current selection for basic and normal are what we want
• [ ] Check if test-local.sh and test-release.sh need updated PYTEST_FLAGS