Add flag to `planemo serve` to save jobs as test cases

Suggested by @mvdbeek in the community call yesterday as a parallel to https://github.com/galaxyproject/planemo/pull/1209 for tool testing.

The idea is you could run something like `planemo serve tool.xml --save-jobs-as-tests` and then any jobs run on the served instance would be saved as test cases (and input and output data downloaded). Maybe added directly to the `` section in the tool wrapper?

Not sure if/when I have time to work on it. Probably it doesn't make sense to start until #1232 is merged anyway.