spring test should not crash on missing test name

When you forget to specify a test name and execute `spring test` you end up with:

```
/Users/senny/Projects/spring/lib/spring/commands.rb:41:in `expand_path': can't convert nil into String (TypeError)
	from /Users/senny/Projects/spring/lib/spring/commands.rb:41:in `call'
	from /Users/senny/Projects/spring/lib/spring/application.rb:76:in `block (2 levels) in serve'
	from /Users/senny/Projects/spring/lib/spring/application.rb:74:in `fork'
	from /Users/senny/Projects/spring/lib/spring/application.rb:74:in `block in serve'
	from /Users/senny/Projects/spring/lib/spring/application.rb:108:in `redirect_output'
	from /Users/senny/Projects/spring/lib/spring/application.rb:64:in `serve'
	from /Users/senny/Projects/spring/lib/spring/application.rb:53:in `block in run'
	from /Users/senny/Projects/spring/lib/spring/application.rb:51:in `loop'
	from /Users/senny/Projects/spring/lib/spring/application.rb:51:in `run'
	from /Users/senny/Projects/spring/lib/spring/application.rb:47:in `start'
	from /Users/senny/Projects/spring/lib/spring/application_manager.rb:74:in `block in start_child'
	from /Users/senny/Projects/spring/lib/spring/application_manager.rb:69:in `fork'
	from /Users/senny/Projects/spring/lib/spring/application_manager.rb:69:in `start_child'
	from /Users/senny/Projects/spring/lib/spring/application_manager.rb:19:in `start'
	from /Users/senny/Projects/spring/lib/spring/application_manager.rb:48:in `block in run'
	from <internal:prelude>:10:in `synchronize'
	from /Users/senny/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/mutex_m.rb:62:in `mu_synchronize'
	from /Users/senny/Projects/spring/lib/spring/application_manager.rb:38:in `run'
	from /Users/senny/Projects/spring/lib/spring/server.rb:39:in `serve'
	from /Users/senny/Projects/spring/lib/spring/server.rb:31:in `block in boot'
	from /Users/senny/Projects/spring/lib/spring/server.rb:31:in `loop'
	from /Users/senny/Projects/spring/lib/spring/server.rb:31:in `boot'
	from /Users/senny/Projects/spring/lib/spring/server.rb:9:in `boot'
	from -e:1:in `<main>'
```

I think we should prevent such expected crashes from happening whenever possible. This is a very straight forward patch and it's very specific to that case. Of course this is more or less a global matter but in my opinion we should let the commands evolve themselves.

Also I try to ramp up or test coverage to prevent regressions.