Changes required to facilitate running of pp_demo_suite using head of dagrunner

[cpelley]

Reverted change from #40 involving common args handling and filled in gap in testing to ensure we continue to support reliant behaviour.

Working pp_demo_workflow when using this dagrunner dev branch: http://fcm1/cylc-review/taskjobs/cpelley?&suite=demo_workflow_dagrunner_head&no_fuzzy_time=0&tasks=execute_basic_model

Best explained from looking at the new testcase:

https://github.com/MetOffice/dagrunner/blob/82788418db7a385af1ba7ed8b57d11e0c7b599f8/dagrunner/tests/execute_graph/test_plugin_executor.py#L71-L73

Issues

• https://github.com/MetOffice/improver_suite/issues/2253