runtime/pprof: make TestBlockProfile less timing dependent

The test for profiling of channel blocking is timing dependent,
and in particular the blockSelectRecvAsync case can fail on a
slow builder (plan9_arm) when many tests are run in parallel.
The child goroutine sleeps for a fixed period so the parent
can be observed to block in a select call reading from the
child; but if the OS process running the parent goroutine is
delayed long enough, the child may wake again before the
parent has reached the blocking point.  By repeating the test
three times, the likelihood of a blocking event is increased.

Fixes #15096

Change-Id: I2ddb9576a83408d06b51ded682bf8e71e53ce59e
Reviewed-on: https://go-review.googlesource.com/21604Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Run-TryBot: Dmitry Vyukov <dvyukov@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/pprof/pprof_test.go

@@ -530,16 +530,21 @@ func blockChanClose() {
 }
 
 func blockSelectRecvAsync() {
+	const numTries = 3
 	c := make(chan bool, 1)
 	c2 := make(chan bool, 1)
 	go func() {
+		for i := 0; i < numTries; i++ {
 			time.Sleep(blockDelay)
 			c <- true
+		}
 	}()
+	for i := 0; i < numTries; i++ {
 		select {
 		case <-c:
 		case <-c2:
 		}
+	}
 }
 
 func blockSelectSendSync() {