Add slash command access to make unit

[bentito]

Description of the change:
This should allow PR comments of the form:
/retest unit to trigger make unit

Motivation for the change:
Prow offers PR comments to trigger fine-grained retest capabilities for tests. This trials adding this capability for the unit test target.

Architectural changes:

None

Testing remarks:

Reviewer Checklist

• Implementation matches the proposed design, or proposal is updated to match implementation
• Sufficient unit test coverage
• Sufficient end-to-end test coverage
• Bug fixes are accompanied by regression test(s)
• e2e tests and flake fixes are accompanied evidence of flake testing, e.g. executing the test 100(0) times
• tech debt/todo is accompanied by issue link(s) in comments in the surrounding code
• Tests are comprehensible, e.g. Ginkgo DSL is being used appropriately
• Docs updated or added to /doc
• Commit messages sensible and descriptive
• Tests marked as [FLAKE] are truly flaky and have an issue
• Code is properly formatted

[m1kola]

I think this change makes merge queue and other events to skip unit tests.

@bentito could you please double check this?

[m1kola]

Looks good to me. I'm approving, but if you could squash into one commit - that would be great.

[bentito]

e2e (3) seems to fail on a "Subscription when unpacking bundle with bundle which OLM will fail to unpack [It] should report deprecated status when catalog is updated to deprecate an installed bundle"?

Seems unrelated to this PR. Merging again as first check on whether this is a flaky e2e test or something that needs fixing to unblock merges.