Add ability to skip running tests on PR

Description of the need

Sometimes a PR makes changes that doesn't require running tests on; e.g. documentation-only changes. In these cases, it'd be nice to save time by being able to skip the automated tests.

Proposed solution

Update our GH action to only run if a certain keyword/phrase isn't present in the PR title/commit message (e.g. [skip ci], [skip tests], etc.). So you'd make your documentation-only PR with the message Issue #12345: Fix typo in README [skip tests], and then the test suite wouldn't run for that PR.

It seems that a simple if condition in the GH action will achieve this: https://github.com/veggiemonk/skip-commit#just-use-yaml-recommended

Alternatives that have been considered

I believe those with the right permissions/access can cancel a workflow, but there are 9 workflows that make up our test suite, so it'd be nice if this was automated instead.

Additional information

Related: https://github.com/backdrop/backdrop-issues/issues/4607

Draft of feature description for Press Release (1 paragraph at most)

Backdrop now allows skipping CI tests in PRs by adding [skip tests] to the PR title.