Combine CI checks into one workflow

[grahamalama]

I think this better-fits the "mental model" of Github actions workflows -- instead of one workflow file per job, each workflow file should represent a, well, workflow e.g. deploy, ci. That workflow may be composed of 1 or more jobs that run to fulfill that workflow.

[grahamalama]

Because we have run_test as a required check, but now the job is called run_unit_tests, I think we'll have to bypass branch protections for this PR (if we decide to merge it), then update the branch protections accordingly

[leplatrem]

That could become a potential quality standard :) I take note of it