Fix/faraday instrumentation test coverage

[aabhinavvvvvvv]

Summary

Improves test coverage for the faraday instrumentation to meet the 85% SimpleCov threshold (fixes #2156).

• Rewrites faraday_test.rb to run under the stable appraisal with tests covering name, version, present?, compatible?, and #install on the instrumentation class
• Adds enable_internal_instrumentation: true test to stable/tracer_middleware_test.rb, old/tracer_middleware_test.rb, and dup/tracer_middleware_test.rb to cover the previously untested else branch in each middleware's call method