add tests for check action, resolve wt cleanup bug

add unit tests for the check action to allow for some
refactoring and bug fixing without having to worry about
breaking stuff.

resolve watchtower cleanup bug by adding an initial 1 second sleep
in the check action. without the sleep, the docker client returns an
empty array, which is why we were left with two watchtowers.

main.go

@@ -106,7 +106,7 @@ func start(c *cli.Context) error {
 		return nil
 	}
 
-	if err := actions.CheckPrereqs(client, cleanup); err != nil {
+	if err := actions.CheckForMultipleWatchtowerInstances(client, cleanup); err != nil {
 		log.Fatal(err)
 	}