runtime: don't skip TestCgoCallbackGC on FreeBSD

Seems to be fixed according to discussion on issue 16396.

Fixes #16396.

Change-Id: Ibac7037a24280204e48cb4d3000af524f65afd36
Reviewed-on: https://go-review.googlesource.com/33903
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/crash_cgo_test.go

@@ -49,8 +49,6 @@ func TestCgoCallbackGC(t *testing.T) {
 	switch runtime.GOOS {
 	case "plan9", "windows":
 		t.Skipf("no pthreads on %s", runtime.GOOS)
-	case "freebsd":
-		testenv.SkipFlaky(t, 16396)
 	}
 	if testing.Short() {
 		switch {