Wrap test framework dependencies

:information_desk_person: The intent of these changes is twofold:
* continuing the work from #16, remove the inline calls to RubyGems and move the test suite dependencies into the helper
* wrap the test framework dependencies into a `Fastly::TestCase` class so that `Test::Unit` can be more easily swapped out for `Minitest`

Following on from these changes, I intend to add unit tests for the library classes themselves to complement the existing integration tests. I'm interested in getting feedback though, so please let me know if these changes are in tune with the project's intent.