muon: only run tests in `passthru.tests`
This was done to for `muonStandalone` to have minimal dependencies.
Other edits:
- Additional cleanup to the postUnpack.
- I've added `cmdlineForMuon` flags that I've learned about since
starting work on the `setupHook`. Without these, we get warnings about
`fortify` requiring `-O2`, and muon defaults to `-O0` when "plain"
isn't given.