os: invoke processKiller synchronously in testKillProcess

Previously, testKillProcess needlessly invoked processKiller in a
separate goroutine and failed to wait for that goroutine to complete,
causing the calls to t.Fatalf in that goroutine to potentially occur
after the test function had already returned.

Fixes #43722

Change-Id: I5d03cb24af51bb73f0ff96419dac57ec39776967
Reviewed-on: https://go-review.googlesource.com/c/go/+/284153


Trust: Bryan C. Mills <bcmills@google.com>
Trust: Jason A. Donenfeld <Jason@zx2c4.com>
Reviewed-by: Jason A. Donenfeld <Jason@zx2c4.com>
Run-TryBot: Jason A. Donenfeld <Jason@zx2c4.com>
TryBot-Result: Go Bot <gobot@golang.org>