Only run the cpu plugin twice when using -test

This can be easily extended to other plugins that need it by tacking
their name onto the switch statement. Also eliminating the unused
TestAllPlugins code and cleaning up some stray Printlns

cmd/telegraf/telegraf.go

@@ -109,16 +109,10 @@ func main() {
 	}
 
 	if *fTest {
-		if *fConfig != "" {
-			err = ag.Test()
-		} else {
-			err = ag.TestAllPlugins()
-		}
-
+		err = ag.Test()
 		if err != nil {
 			log.Fatal(err)
 		}
-
 		return
 	}