net: use testenv.SkipFlaky instead of testing.Skip

Change-Id: Ic219fedbe6bbb846f31111fa21df6f2b8620e269
Reviewed-on: https://go-review.googlesource.com/33263
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/timeout_test.go

@@ -151,7 +151,7 @@ var acceptTimeoutTests = []struct {
 }
 
 func TestAcceptTimeout(t *testing.T) {
-	t.Skip("flaky test; golang.org/issue/17948")
+	testenv.SkipFlaky(t, 17948)
 	t.Parallel()
 
 	switch runtime.GOOS {