Projectile: Migrate the tests from ERT to Buttercup

[bbatsov]

I've decided that I'm done with ERT, as it's really painful to work with it, and I want us to migrate all the tests to Buttercup.

I've renamed the old tests to projectile-legacy-test.el and projectile-test.el already contains a couple of migrated tests. make test now runs the new tests and make ert-test runs the old ones.

I'd really appreciate some help in migrating all the tests and generally increasing the code coverage of the project. I think all of you will find it's much easier to work with buttercup (especially if you've ever used something like RSpec or Jasmine).

[mnuessler]

Nice, I would like to work on this..

[bbatsov]

@mnuessler You've got it. :-)