Add withTimeout() to surface hangs as diagnosable failures

[broken-circle]

This PR adds withTimeout() as an internal test helper and applies it to testDoesNotReapUnrelatedChildProcess(), so a hang surfaces as a TimeoutError with the test name and timeout duration instead of consuming the full CI job timeout with no actionable diagnostic.

withTimeout() is adapted from the swift-build implementation as suggested by @jakepetroules. I made description non-optional since every call site in this repo is likely to want a useful description, added defer-based cancelation, and threw directly from group.next(). I can use the exact swift-build version if you'd prefer it.

Addresses #157, which should remain open until the reason for the underlying hang is determined.

[jakepetroules]

I actually don't remember why I suggested using the Swift Build code as Swift Testing has had the testTimeout trait for well over a year. @iCharlesHu do you remember what the issue was here?

[broken-circle]

Yeah, .timeLimit(.minutes(1)) is simpler. I can update the PR to use that and drop the helper. One thing worth noting is the coarse granularity:

Test timeouts do not support high-precision, arbitrarily short durations due to variability in testing environments. You express the duration in minutes, with a minimum duration of one minute.

The 1 minute timeout is fine for this test, but if finer-grained timeouts come up later, the helper would be more useful. I'd say we can drop it now and come back to it later if we need it.