Re-enable NATS test with higher timeout

[evankanderson]

Summary

Re-enable tests for the NATS messaging channel; running this with go test -count 200 ./internal/events/nats produced 5 failures with the previous 5s timeout. Increasing the timeout to 15s avoided all timeouts.

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 -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.

[coveralls]

coverage: 53.259% (+0.6%) from 52.658%
when pulling 98233ea on evankanderson:fix-nats-tests
into 87aae86 on stacklok:main.