Ensure --mets is properly set in bashlib

[kba]

We should probably also add some basename tests with the dummy processor (like test_mets_basename and test_ocrd_dummy.TestDummyProcessor combined).

Also, we have next to no tests for bashlib processing yet.

BTW, before ocrd-olena-binarize (and the rest of them) can actually handle the non-default basename case correctly, we would still have to rewrite all the ocrd workspace calls to something like ocrd workspace -m $(basename ${ocrd__argv[mets_file]}).

And so to facilitate that, in turn, we should probably add something like ocrd__workspace function to lib.bash (which would work both before and after the usual cd call)...

Originally posted by @bertsky in #925 (review)

[bertsky]

This ocrd__workspace could also do what is needed for #938 – i.e. downloading locally (without changing the href).