Replace some workflow tests with Node test suite

[Bo98]

A number of our workflows use write tokens for integration tests. This isn't great and prevents third-party contributions, so let's replace that with tests that we can mock API calls and other benefits such as code coverage monitoring.

Replaced tests

Action	Status
check-commit-format	✅ Could do with better coverage but is no worse than what we had before
dismiss-approvals	✅
label-pull-requests	✅ Could do with better coverage but is no worse than what we had before
post-comment	✅

Still integration tests, but in good state

Action	Status
brew-script	Composite action that executes Ruby. Unchanged as the existing workflow doesn't need sensitive tokens
git-try-push	Executes commands so messier to mock. Need for write tokens when testing removed in #541
git-user-config	Executes commands. Unchanged as the existing workflow doesn't need sensitive tokens
setup-commit-signing	Shell script so can't be integrated into the Node test suite. Need for secrets when testing removed in #540
setup-homebrew	Shell script. Unchanged as the existing workflow doesn't need sensitive tokens and is fairly comprehensive as is

Note that these will not have code coverage monitored as a result.

No tests; need attention

Action	Status	Recommendation
bump-formulae	Deprecated but stable. Any bump-package test coverage would cover this one	Add tests to bump-packages
bump-packages	Composite action. No tests	Figure out how to test this
create-gcloud-instance	Shell scripts. No tests	Will deprecate this soon
failures-summary-and-bottle-result	Composite action. No tests	Figure out how to test this
find-related-workflow-run-id	Uses gh. No tests	Migrate this to use octokit.js
limit-pull-requests	Uses gh. No tests	Migrate this to use octokit.js
post-build pre-build	Composite action. No tests and has a history of "blind changes"	Move this to homebrew-core
remove-disabled-formulae	Uses Ruby, executes commands. No tests	Figure out how to test this
wait-for-idle-runner	Can easily be added to the Node test suite but isn't expected to stay around for long	Will deprecate this soon

[MikeMcQuaid]

Nice work, makes sense to me! Haven't done detailed review of the NodeJS code but approach is sensible.