A Project root can be defined by the presence of a test directory

This is useful for projects where a Rakefile is not present, as it provides an alternative means of detecting whether your project root is actually your project root.

I ran into this issue when working on a Goliath project which had none of the usual trappings of a Rakefile, Rails config files or a gemspec. Ruby test mode was causing Emacs to hang when running the specs unless I a) applied this patch, or b) touched an empty `Rakefile` in the root of the project.

I've built a test project that can be used to test this functionality. You can find it here:

https://github.com/eightbitraptor/ruby-test-mode-issue-example