runtime: no longer skip stack growth test in short mode

We originally decided to skip this test in short mode
to prevent the parallel runtime test to timeout on the
Plan 9 builder. This should no longer be required since
the issue was fixed in CL 86210043.

LGTM=dave, bradfitz
R=dvyukov, dave, bradfitz
CC=golang-codereviews, rsc
https://golang.org/cl/84790044

src/pkg/runtime/stack_test.go

@@ -123,9 +123,6 @@ func TestStackMem(t *testing.T) {
 
 // Test stack growing in different contexts.
 func TestStackGrowth(t *testing.T) {
-	if testing.Short() {
-		t.Skip("skipping test in short mode")
-	}
 	t.Parallel()
 	var wg sync.WaitGroup