CI: smoke test: insist on adding tests on PRs

On each PR (with a few exceptions), check the list of git-touched
files, and abort if no tests are added. Include instructions
on how to bypass the check if tests really aren't needed.

Include a hardcoded exception list for PRs that only touch a
well-known subset of "safe" files: docs, .cirrus.yml, vendor,
version, hack, contrib, or *.md. This list is likely to need
tuning over time.

Add a test suite, but not one recognized by the new script
(because it's a "*.t" file), so: [NO TESTS NEEDED]

Signed-off-by: Ed Santiago <santiago@redhat.com>

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