Actions lacking tests

[Bo98]

(Extracted from #542)

The following actions are lacking tests and thus we are currently blindly merging changes to:

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	@Bo98 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. Probably the most actively maintained in this list	Move this to homebrew-core, which already has CI coverage
remove-disabled-packages	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	@Bo98 will deprecate this soon

Node.js tests are preferred (see *.test.mjs files) as it links with Codecov.
Dedicated test workflows (like setup-homebrew) are acceptable if Node.js tests are demonstrated to not be possible/feasible (e.g. things written in pure shell that don't benefit from porting to JS, such as bump-packages). Any such test workflows however should not require any write tokens or secrets.

[MikeMcQuaid]

Thanks for opening this @Bo98!