Allow usage of marks with brakets

[ChristophSchwenk]

We are using the pytest xdist plugin to run tavern tests in paralell. To assure certain tests are not running in parallel but in sequence we want to use the xdist --dist loadgroup feature. To do so we need to mark our tests with something like this:

@pytest.mark.xdist_group("group1") @pytest.mark.xdist_group("group2")

However it seems that tavern is not adding marks with brakets correctly.

Adding:

mark: - xdist_group("group1")

Seems not to have the wanted effect.