Refactor test suite

[tombruijn]

• Use a random order for the test suite. Spot problems with connected tests.
• Don't unit test integration(s middleware), but test against an actual app. See Padrino tests for an example.
• Refactor out testing private methods
• Less stubbing and mocking
• Test transaction contents from extension.
• Less before :all|context usage
• Less instance variables usage
• Fix env var tests: Fix configuring instrument redis and sequel from env #257 (review)
• spec/lib/appsignal_spec.rb, many spec with expect do .. end.to_not raise_error useless checks. Spec fails if an error occurs
• Clean up test output -> Explicit header whitelist in configuration #406 (comment)
  • If logs are printed in the test output that are important to see, we should test their presence
• Remove "duplicate" builds from Travis
  • We now test against a lot of combinations of Ruby versions and gems, but I think we can decrease the amount a bit. Decrease/Drop the 2.1.x-2.3.x builds for all combinations of gems/only against Rails and no-deps. Speeds the build up quite a bit.
• Add macOS to the build on Semaphore.