Allow more than one order marker per test

[mrbean-bremen]

Currently, only one order marker per test is allowed. This does not matter if the markers are directly on the test, as the attributes can always be written i the same marker, but it does if there are both class and test markers shall be used, e.g.:

@pytest.mark.order(after="TestClass2")
class TestClass1:
    @pytest.mark.order(2)
    def test_1():
        pass

    @pytest.mark.order(1)
    def test_2():
        pass

will currently not work, because the class marker will be ignored if a local test marker is present.

[mrbean-bremen]

This is actually a bug - the documentation notes that class markers work, so they also have to work with test markers.