regexp: skip TestOnePassCutoff in short mode

Runs for 4 seconds on my mac.
Also this is the only test that times out on freebsd in -race mode.

R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/110150045

src/pkg/regexp/all_test.go

@@ -475,6 +475,9 @@ func TestSplit(t *testing.T) {
 
 // This ran out of stack before issue 7608 was fixed.
 func TestOnePassCutoff(t *testing.T) {
+	if testing.Short() {
+		t.Skip("Skipping in short mode")
+	}
 	MustCompile(`^(?:x{1,1000}){1,1000}$`)
 }