tests: Assert that test_missing_arch gives a useful diagnostic
The message is actually something like
/path/to/hal9000-linux-gnu-inspect-library not found
but I don't want to hard-code that, so just assert that the name of the executable is mentioned.
This clarifies why we don't get an exit status: we never get as far as actually running anything.
Non-essential follow-up for !87 (merged); it seemed quicker to merge !87 (merged) and implement this myself than to do another review cycle. /cc @jpwhiting @denittis