Add homedir to plist search path to enable agents provided by the user

I tried to launch a custom per-user agent as the current user with the launchd service provider but failed. My plist was located at ```~/Library/LaunchAgents/my.custom.service.plist```. The cause was that the home dir is not part of the pattern to find the plist file. I also added some tests for the start method of the service provider.
@skrings What do you think about @dgoodlad's comment from above? I'd really like to see this merged so that I can use it.