runtime/trace: deflake TestTraceSymbolize

Waiting 2ms for all the kicked-off goroutines to run and block
seems a little optimistic. No harm done by waiting for 200ms instead.

Fixes #17238.

Change-Id: I827532ea2f5f1f3ed04179f8957dd2c563946ed0
Reviewed-on: https://go-review.googlesource.com/32103
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/trace/trace_stack_test.go

@@ -102,10 +102,10 @@ func TestTraceSymbolize(t *testing.T) {
 		pipeReadDone <- true
 	}()
 
-	time.Sleep(time.Millisecond)
+	time.Sleep(100 * time.Millisecond)
 	runtime.GC()
 	runtime.Gosched()
-	time.Sleep(time.Millisecond) // the last chance for the goroutines above to block
+	time.Sleep(100 * time.Millisecond) // the last chance for the goroutines above to block
 	done1 <- true
 	<-done2
 	select {