Add test workflow with unittest

[smokestacklightnin]

Changes in this PR:

• Add .gitignore file
• Add unit test GitHub workflow
• Add unittest skips or expectedFailures to tests with errors and failures, respectively

You can get a list of tests with skips or expectedFailures using

$ grep -nr "PR 189" tensorflow_model_analysis

There is one import error that I have not dealt with.

[smokestacklightnin]

@embr This PR seems to me to be ready for review and merge. It just adds a testing workflow. I think fixing the failing tests can come from subsequent work.

The test failure is because of an import error. The import refers to code that is "in maintenance mode" and "not available in TFMA OSS". Until the code becomes present in tfma.eval_saved_model, the tests will not pass.

[peytondmurray]

I'm going to update the branch. If we have failing tests, let's mark them as XFAIL here, then make a PR to remove the missing imports - now that this is officially community maintained, we shouldn't keep tests around for code we will never see here.

[smokestacklightnin]

@peytondmurray Tests are all passing or xfailed!

Will clean up the PR and ping you for review