test(complete): Unblock E2E dynamic completion tests

[epage]

This was split out of #5048 to iterate on some ideas to deal with the challenges involved

• I ran into issues with the test command (I assume that is why feat(completions): Dynamic fish completions #5048 had testcomp). I chose exhaustive to describe what it does.
• Don't use separate target directories but have the binaries work as one
  • This meant dealing with the static tests needing to generate the same result, independent of unstable-dynamic, so I made the tests always enable it

I held off on adding the bash completion test because the order varies between my machine and CI.

[ModProg]

This looks definitely better.