runtime: usleep in TestWeakToStrongMarkTermination

There's a subtle bug in this test (big surprise): time.Sleep allocates,
so the time.Sleep(100*time.Millisecond) before unblocking gcMarkDone
might itself end up in gcMarkDone.

Work around this by using usleep here instead.

Fixes #70532.

Change-Id: I4c642ebb12f737cdb0d79ccff64b6059fc3d8b34
Reviewed-on: https://go-review.googlesource.com/c/go/+/636155
Reviewed-by: Cherry Mui <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: mknyszek

src/runtime/gc_test.go

@@ -834,7 +834,11 @@ func TestWeakToStrongMarkTermination(t *testing.T) {
 		done <- struct{}{}
 	}()
 	go func() {
-		time.Sleep(100 * time.Millisecond)
+		// Usleep here instead of time.Sleep. time.Sleep
+		// can allocate, and if we get unlucky, then it
+		// can end up stuck in gcMarkDone with nothing to
+		// wake it.
+		runtime.Usleep(100000) // 100ms
 
 		// Let mark termination continue.
 		runtime.SetSpinInGCMarkDone(false)