Add more tests

[CasperWA]

We should add tests for:

• The error handlers. Testing ErrorResponse and all of the different Exception handlers.
  • Partly implemented by Introduce grammar v0.10.1 #112 with 71ab66c.
  • More implemented in Implement include query parameter #163.
• The validator. Run client and run validator. Test Client in validator, etc.
  • the CLI
  • validating mandatory filter examples from spec (done in Validate all non-optional :filter: examples from the spec #213)
  • validating optional filter examples from spec
• response_fields is not tested.
  • Fixed by New REQUIRED level properties #153.
• sort is not tested.
  • May be implemented by Use pytest instead of unittest #390
• Parse JSON config-file.
  • Implemented by Fix load_from_json #137
• Field validation edge-cases, making sure the regex for the query parameters catch all
• Landing page.

[ml-evs]

I think I'm going to close this issue, as we have certainly written a lot of tests since it was opened.