Stop skipping multi-sweep masking tests

[benjaminhwilliams]

Some tests exist for masking multi-sweep data but they have been marked as skipped in the absence of test data in dials/data. Now that those data exist, we can enable the tests.

Also move from dxtbx.serialize.load to dxtbx.ExperimentList.from_file.

I suspect my somewhat imperfect pytest-fu means I've created a rather cumbersome parametrised fixture here — I might benefit from someone else's eyes on this in particular.

In short, before:

$ pytest --regression -k test_generate_mask
...
 test/command_line/test_generate_mask.py ✓s✓s✓s✓s✓s✓s✓s✓s✓✓s           100% ██████████
============================== short test summary info ===============================
SKIPPED [9] test/command_line/test_generate_mask.py: test depends on https://github.com/dials/data/pull/40

Results (5.18s):
      10 passed
       9 skipped
    1050 deselected

After:

$ pytest --regression -k test_generate_mask
...
 test/command_line/test_generate_mask.py ✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓           100% ██████████

Results (8.06s):
      19 passed
    1050 deselected

[benjaminhwilliams]

Sorry, branched before a commit on master that presumably fixed the failing cosym test. Hopefully a merge commit will make green ticks happen...