[TRAVIS POC] Using platforms/dockerfiles/mac.sh as source of truth for generating Build, Parallel, Serial, and LRTs

[NorseGaud]

Problem

Mac does not work inside of docker. We have to install dependencies on each run and that's painfully long. In order to interpolate the hash of the macos-10.14.sh script into the various steps, and avoid having to manually set them, I'd like to move to generating more than just serial tests programmatically. This allows us to obtain the hash once and then interpolate it in the various places we need it.

This also solves:

1. Having to set IMAGE_TAG env in the pipeline.yml manually to the name of the dockerfile.
2. Ensure step (with hashed tag name) can now be added easily using the same logic we use for other steps.
3. Support for PINNED=false/UNPINNED=true being set and having to use the {platform}-unpinned tags (elimination of the unpinned.yml and code from the pipeline settings).
4. Elimination of complicated docker hash function into a simpler helper.
5. Elimination of generate-serial-tests.sh
6. Long Running Tests are split out into "container level parallelization"

The pipeline has a generate step:

After the generate step runs, it will show a complete pipeline:

Tested

• Buildkite beta
  • Build 1732 — pinned (base case)
  • Build 1733 — unpinned
  • Build 1734 — long-running tests
• Travis CI build 4200

See Also

• Pull Request 7696 — CI/CD work
• Pull Request 7831 — test script changes, merged into this branch

We merged in my commits from PR7831, so I am a contributor to this pull request now. Someone else will have to review.

[jerstern]

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