Add OME sample data to tests, and mark with xfail for now

[tlambert03]

This PR adds all of the sample xml files from https://github.com/ome/ome-model/tree/master/specification/samples/2016-06 to the testing suite, and explicitly marks the files that are not known to pass with pytest.mark.xfail. As such, you can now run pytest and it will build the model, and test all of the samples, and if you fix a problem the test will actually fail with:
[XPASS(strict)] Unexpected success. You fixed it!

Then, add the name of the file(s) that your changes have fixed to the SHOULD_PASS set in test_autogen.py. Currently this only includes:

SHOULD_PASS = {"example.ome", "small.ome"}
SHOULD_RAISE = {"bad.ome"}  # this xml is actually invalid and should raise validation error.

[codecov-commenter]

Codecov Report

Merging #10 into master will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #10      +/-   ##
==========================================
+ Coverage   95.37%   95.40%   +0.03%     
==========================================
  Files           1        1              
  Lines         389      392       +3     
==========================================
+ Hits          371      374       +3     
  Misses         18       18              

Impacted Files	Coverage Δ
src/ome_autogen.py	95.40% <100.00%> (+0.03%)	⬆️

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[tlambert03]

@jmuhlich, this should help iterate quicker locally on #9

[jmuhlich]

Great idea!