Rollup merge of #47986 - Gilnaa:libtest_relaxed, r=Mark-Simulacrum

libtest: Replace panics with error messages

This replaces explicit panics on failures in libtest with prints to stderr.
Where "failures" == CLI argument parsing and such

Before:
```
$ ./foo-stable --not-an-option
thread 'main' panicked at '"Unrecognized option: \'not-an-option\'"', libtest/lib.rs:251:27
note: Run with `RUST_BACKTRACE=1` for a backtrace.
```

After:
```
$ ./foo-nightly --not-an-option
error: Unrecognized option: 'not-an-option'
```