go_1_9: skip flaky TestWaitGroupMisuse2

https://github.com/golang/go/issues/20072
https://hydra.nixos.org/build/64351217/nixlog/1

pkgs/development/compilers/go/1.9.nix

@@ -119,6 +119,7 @@ stdenv.mkDerivation rec {
       ./creds-test.patch
       ./remove-test-pie-1.9.patch
       ./go-1.9-skip-flaky-19608.patch
+      ./go-1.9-skip-flaky-20072.patch
     ];
 
   postPatch = optionalString stdenv.isDarwin ''

pkgs/development/compilers/go/go-1.9-skip-flaky-20072.patch

@@ -0,0 +1,20 @@
+diff --git a/src/sync/waitgroup_test.go b/src/sync/waitgroup_test.go
+index e3e3096..f80d1e2 100644
+--- a/src/sync/waitgroup_test.go
++++ b/src/sync/waitgroup_test.go
+@@ -6,6 +6,7 @@ package sync_test
+ 
+ import (
+ 	"internal/race"
++	"internal/testenv"
+ 	"runtime"
+ 	. "sync"
+ 	"sync/atomic"
+@@ -73,6 +74,7 @@ func TestWaitGroupMisuse2(t *testing.T) {
+ 	if runtime.NumCPU() <= 4 {
+ 		t.Skip("NumCPU<=4, skipping: this test requires parallelism")
+ 	}
++	testenv.SkipFlaky(t, 20072)
+ 	defer func() {
+ 		err := recover()
+ 		if err != "sync: negative WaitGroup counter" &&