Allow running tests without a bundle

[vinistock]

Running for default frameworks, such as Minitest or Test Unit, is not working if you don't have a Gemfile.

[vinistock]

This problem is not so easy to fix. We don't automatically index gems that aren't in the Gemfile and so we have no way to discover test items because we never indexed Minitest::Test or Test::Unit::TestCase.

We may need to make some different decisions to improve support for tests without a bundle, although I do feel like requiring people to add their test framework to the Gemfile is a fair requirement.