sync: don't run known-racy tests under the race detector

Fixes the build from https://golang.org/cl/4117 (sync: simplify WaitGroup)

Change-Id: Icc2a7ba8acea26fd187d52cf1901bfebf8573f93
Reviewed-on: https://go-review.googlesource.com/11591Reviewed-by: Austin Clements <austin@google.com>

src/sync/export_test.go

@@ -7,3 +7,5 @@ package sync
 // Export for testing.
 var Runtime_Semacquire = runtime_Semacquire
 var Runtime_Semrelease = runtime_Semrelease
+
+const RaceEnabled = raceenabled

src/sync/waitgroup_test.go

@@ -47,6 +47,12 @@ func TestWaitGroup(t *testing.T) {
 	}
 }
 
+func knownRacy(t *testing.T) {
+	if RaceEnabled {
+		t.Skip("skipping known-racy test under the race detector")
+	}
+}
+
 func TestWaitGroupMisuse(t *testing.T) {
 	defer func() {
 		err := recover()
@@ -62,6 +68,7 @@ func TestWaitGroupMisuse(t *testing.T) {
 }
 
 func TestWaitGroupMisuse2(t *testing.T) {
+	knownRacy(t)
 	if runtime.NumCPU() <= 2 {
 		t.Skip("NumCPU<=2, skipping: this test requires parallelism")
 	}
@@ -104,6 +111,7 @@ func TestWaitGroupMisuse2(t *testing.T) {
 }
 
 func TestWaitGroupMisuse3(t *testing.T) {
+	knownRacy(t)
 	if runtime.NumCPU() <= 1 {
 		t.Skip("NumCPU==1, skipping: this test requires parallelism")
 	}