* Add a new test to catch exit code failure
fixes#29226
This introduces a new unit test that checks the return
code of `spack unit-test` when it is supposed to fail.
This is to prevent bugs like the one introduced in #25601
in which CI didn't catch a missing return statement.
In retrospective it seems that the shell test we have right
now all go through `tty.die` or similar code paths which
call `sys.exit(a)` explicitly. This new test instead checks
`spack unit-test` which relies on the return code from
command invocation in case of errors.
9b298fd7e4