misc/cgo/test: skip test18146 on Darwin

It is reported as failing for two people (issues #18202 and #18212).
The failure mode is that the system gets overloaded and other programs
fail to run.

Fixes #18202.

Change-Id: I1f1ca1f5d8eed6cc3a9dffac3289851e09fa662b
Reviewed-on: https://go-review.googlesource.com/34017
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

misc/cgo/test/issue18146.go

@@ -22,6 +22,10 @@ import (
 )
 
 func test18146(t *testing.T) {
+	if runtime.GOOS == "darwin" {
+		t.Skipf("skipping flaky test on %s; see golang.org/issue/18202", runtime.GOOS)
+	}
+
 	attempts := 1000
 	threads := 4