Reenable tests

[karlb]

After merging the arbitrum changes, some tests were skipped to make the rest of the test suite run again. Unskip and fix these tests.

$ ag 'Broken by arbitrum changes'
raiden_common/tests/unit/utils/test_filters.py
14:@pytest.mark.skip(reason="Broken by arbitrum changes")

raiden_common/tests/unit/test_pfs_integration.py
248:@pytest.mark.skip(reason="Broken by arbitrum changes")
279:@pytest.mark.skip(reason="Broken by arbitrum changes")
515:@pytest.mark.skip(reason="Broken by arbitrum changes")
573:@pytest.mark.skip(reason="Broken by arbitrum changes")
631:@pytest.mark.skip(reason="Broken by arbitrum changes")

Do this after #9, so that we don't spend time on fixing tests for code we are about to remove.