Rollup merge of #142807 - sourcefrog:failfast, r=dtolnay

libtest: expose --fail-fast as an unstable command-line option

This exposes the `fail_fast` option added in rust-lang/rust#105153 on the test harness command line, so that workflows that only want to know if any test fails can find out without waiting for everything to run. For example, cargo-mutants just needs to know if any tests fails. It only works with `-Zunstable-options`.

Tracking issue: rust-lang/rust#142859

Author: Zalathar