Simplify ipa-run-tests script
    
    This is a sort of rollback to the pre #93c158b05 state with
    several improvements.
    
    For now, the nodeids calculation by ipa-run-tests is not stable,
    since it depends on current working directory. Nodeids (tests
    addresses) are utilized by the other plugins, for example.
    
    Unfortunately, the `pytest_load_initial_conftests` hook doesn't
    correctly work with pytest internal paths, since it is called
    after the calculation of rootdir was performed, for example.
    
    Eventually, it's simpler to follow the default convention for
    Python test discovery.
    
    There is at least one drawback of new "old" implementation.
    The ignore rules don't support globs, because pytest 4.3.0+
    has the same facility via `--ignore-glob`:
    
    > Add the `--ignore-glob` parameter to exclude test-modules with
    > Unix shell-style wildcards. Add the collect_ignore_glob for
    > conftest.py to exclude test-modules with Unix shell-style
    > wildcards.
    
    Upon switching to pytest4 it will be possible to utilize this.
    Anyway, tests for checking current basic facilities of
    ipa-run-tests were added.
    
    Fixes: https://pagure.io/freeipa/issue/8007
    Signed-off-by: Stanislav Levin <slev@altlinux.org>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>