Further harden NATS test, accept duplicate deliveries and out-of-order better

[evankanderson]

Summary

It looks like the NATS tests would sometimes time out or have other issues under the "Coverage" legs (but not elsewhere).

Running go test -race -cover -count 200 ./internal/events/nats I was able to find a few more failing cases and terminate them with prejudice.

Change Type

Mark the type of change your PR introduces:

• Bug fix (resolves an issue without affecting existing features)
• Feature (adds new functionality without breaking changes)
• Breaking change (may impact existing functionalities or require documentation updates)
• Documentation (updates or additions to documentation)
• Refactoring or test improvements (no bug fixes or new functionality)

Testing

go test -race -cover -count 200 ./internal/events/nats

Review Checklist:

• Reviewed my own code for quality and clarity.
• Added comments to complex or tricky code sections.
• Updated any affected documentation.
• Included tests that validate the fix or feature.
• Checked that related changes are merged.

[evankanderson]

Using a Fatal here means that if we don't get redeliveries, we may error out on the tests. I'm not sure why I didn't see this locally, but did see this on CI. CI is correct.

[coveralls]

coverage: 53.241% (-0.4%) from 53.661%
when pulling dd900c4 on evankanderson:really-fix-nats-test
into 7d6a2e0 on stacklok:main.