Add separate integration tests

[postmodern]

Add proper integration tests which run chruby against /opt/rubies or a given rubies/ directory. The test script should directly query the ruby for it's settings (RUBY_VERSION, RUBY_ENGINE, etc), attempt to switch to each ruby in the rubies/ directory, validate that the appropriate ENV variables have been set correctly, explicitly error on empty-values (ex: if the ruby returns an empty string for GEM_ROOT or RUBY_VERSION, something has broken).

All unit tests shall be moved into test/unit/, and all integration tests shall reside in test/integration/. Integration tests should probably not run as part of the CI just yet, and are more intended for local testing.

[postmodern]

Implemented in a02854f and f8f29a5.