runtime/pprof: deflake TestGoroutineCounts

TestGoroutineCounts currently depends on timing to get 100 goroutines
to a known blocking point before taking a profile. This fails
frequently, with different goroutines captured at different stacks.
The test is disabled on openbsd because it was too flaky, but in fact
it flakes on all platforms.

Fix this by using Gosched instead of timing. This is both much more
reliable and makes the test run faster.

Fixes #15156.

Change-Id: Ia6e894196d717655b8fb4ee96df53f6cc8bc5f1f
Reviewed-on: https://go-review.googlesource.com/42953
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/pprof/pprof_test.go

@@ -575,22 +575,21 @@ func func3(c chan int) { <-c }
 func func4(c chan int) { <-c }
 
 func TestGoroutineCounts(t *testing.T) {
-	if runtime.GOOS == "openbsd" {
-		testenv.SkipFlaky(t, 15156)
-	}
 	c := make(chan int)
 	for i := 0; i < 100; i++ {
-		if i%10 == 0 {
+		switch {
+		case i%10 == 0:
 			go func1(c)
-			continue
-		}
-		if i%2 == 0 {
+		case i%2 == 0:
 			go func2(c)
-			continue
+		default:
+			go func3(c)
+		}
+		// Let goroutines block on channel
+		for j := 0; j < 5; j++ {
+			runtime.Gosched()
 		}
-		go func3(c)
 	}
-	time.Sleep(10 * time.Millisecond) // let goroutines block on channel
 
 	var w bytes.Buffer
 	goroutineProf := Lookup("goroutine")