(#52) Shared CI workflow for APIs.

[blairlearn]

This is the foundational commit for having a single workflow file among the APIs.

These PRs depend on this one being merged and released.

• NCIOCPL/glossary-api#177
• NCIOCPL/drug-dictionary-api#95
• NCIOCPL/sitewide-search-api#110
• NCIOCPL/bestbets-api#61
• NCIOCPL/r4r-api#29
• NCIOCPL/clinical-trials-listing-api#66

Closes #52 Closes #56

[blairlearn]

@jfrank-nih

• Workflow concurrency: Totally worthwhile. It's added.
• Uploading integration results: I'm going to hold off on that. Integration tests can (and should!) be run locally before submitting a PR.
• Running unit tests on Mac: As you said, it doesn't hurt, and we do our development on Mac, so this is an extra check to make sure they run there.
• Not saving code coverage outputs on Windows: For whatever reason, on Windows, the coverage tests get confused about the number of test projects being specified (only one exists) and the tests fail. I've added a comment to explain why Windows is being treated differently.