runtime/pprof: disable TestTraceStressStartStop

Updates #10476

Change-Id: Ic4414f669104905c6004835be5cf0fa873553ea6
Reviewed-on: https://go-review.googlesource.com/8962
Reviewed-by: Russ Cox <[email protected]>

Author: davecheney

src/runtime/pprof/trace_test.go

@@ -229,6 +229,7 @@ func TestTraceStress(t *testing.T) {
 // And concurrently with all that start/stop trace 3 times.
 func TestTraceStressStartStop(t *testing.T) {
 	skipTraceTestsIfNeeded(t)
+	t.Skip("test is unreliable; issue #10476")
 
 	defer runtime.GOMAXPROCS(runtime.GOMAXPROCS(8))
 	outerDone := make(chan bool)