crypto/x509: re-enable TestSystemRoots on darwin

It was apparently waiting on CL 36942, which was submitted.

Fixes #21416

Change-Id: I8f4ccc5a3176070abf0df019c82700c5761b5f53
Reviewed-on: https://go-review.googlesource.com/117055
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/crypto/x509/root_darwin_test.go

@@ -16,11 +16,6 @@ func TestSystemRoots(t *testing.T) {
 		t.Skipf("skipping on %s/%s, no system root", runtime.GOOS, runtime.GOARCH)
 	}
 
-	switch runtime.GOOS {
-	case "darwin":
-		t.Skipf("skipping on %s/%s until cgo part of golang.org/issue/16532 has been implemented.", runtime.GOOS, runtime.GOARCH)
-	}
-
 	t0 := time.Now()
 	sysRoots := systemRootsPool() // actual system roots
 	sysRootsDuration := time.Since(t0)