runtime/pprof: skip TestStackBarrierProfiling on dragonfly too

Just saw a few dragonfly failures here.

I'm tempted to preemptively add plan9 here too, but I'll wait until
I see it fail.

Change-Id: Ic99fc088dbfd1aa21f509148aee98ccfe7f640bf
Reviewed-on: https://go-review.googlesource.com/18306Reviewed-by: Russ Cox <rsc@golang.org>

src/runtime/pprof/pprof_test.go

@@ -333,7 +333,7 @@ func TestMathBigDivide(t *testing.T) {
 }
 
 func TestStackBarrierProfiling(t *testing.T) {
-	if (runtime.GOOS == "linux" && runtime.GOARCH == "arm") || runtime.GOOS == "openbsd" || runtime.GOOS == "solaris" {
+	if (runtime.GOOS == "linux" && runtime.GOARCH == "arm") || runtime.GOOS == "openbsd" || runtime.GOOS == "solaris" || runtime.GOOS == "dragonfly" {
 		// This test currently triggers a large number of
 		// usleep(100)s. These kernels/arches have poor
 		// resolution timers, so this gives up a whole