Add code coverage to buildkite

It would be really nice if we could get a more accurate picture of code coverage in our unit tests, on buildkite. I think doing this, and improving test coverage will give us confidence to move our examples out of this repo, and fully rely on slimmer, more rigorous, unit tests.

No files or entry points are named. Start by locating the Buildkite configuration and the Julia unit-test setup, then determine how coverage is currently generated, if at all. Done means Buildkite reports an accurate coverage result for the unit tests and the result can guide moving examples out of the repository.

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