runtime: execute TestSpuriousWakeupsNeverHangSemasleep in parallel

This test spends most of its time sleeping and waiting on a subprocess
to sleep. It seems like a prime candidate to run in parallel, although
we may need to relax its hard-coded 2s timeout on the sleep(1)
subprocess.

For #48770

Change-Id: I4e839739fe82446615f9894c1904c87e5f3cf386
Reviewed-on: https://go-review.googlesource.com/c/go/+/366256
Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>

Author: Bryan C. Mills

src/runtime/semasleep_test.go

@@ -21,6 +21,7 @@ func TestSpuriousWakeupsNeverHangSemasleep(t *testing.T) {
 	if *flagQuick {
 		t.Skip("-quick")
 	}
+	t.Parallel() // Waits for a program to sleep for 1s.
 
 	exe, err := buildTestProg(t, "testprog")
 	if err != nil {