Rollup merge of #81546 - hyd-dev:libtest-run-out-of-threads, r=Mark-Simulacrum

Rollup merge of #81546 - hyd-dev:libtest-run-out-of-threads, r=Mark-Simulacrum

[libtest] Run the test synchronously when hitting thread limit

libtest currently panics if it hits the thread limit. This often results in spurious test failures (<code>thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Os { code: 11, kind: WouldBlock, message: "Resource temporarily unavailable" }'</code> ... `error: test failed, to rerun pass '--lib'`). This PR makes it continue to run the test synchronously if it runs out of threads.

Closes #78165.

``@rustbot`` label: A-libtest T-libs