Skip long light tests in -short mode

I added checks against testing.Short for tests that took longer than
about one second on my machine. Running with -short reduces the test
time of light from about 21s to about 0.9s.

light/example_test.go

@@ -17,6 +17,10 @@ import (
 
 // Manually getting light blocks and verifying them.
 func TestExampleClient(t *testing.T) {
+	if testing.Short() {
+		t.Skip("skipping test in short mode")
+	}
+
 	ctx, cancel := context.WithCancel(context.Background())
 	defer cancel()
 	conf, err := rpctest.CreateConfig(t, "ExampleClient_VerifyLightBlockAtHeight")