Clean up the test framework

[nhtruong]

This repo has received 2 major overhauls in the past 2 years (Merging gems, and refactoring API code along with a new API codegen) but the test framework has not been cleaned up to removed artifacts that are not longer relevant. A few things that we can do right now:

• remove unused gems in gemfile
• a better strategy to test different combinations of faraday and its adapters.
• remove tests that are no longer relevant (and adding new ones if necessary)