Refactoring IntegrationTestCase class:

  - moved all API URL generation logic & API response handling logic to separate classes
  - created ApiTestConfig class to hold API test options (and documentation for properties)
  - remove unnecessary IntegrationTestCase properties and methods (including apiNotToCall/apiToCall)
  - remove as many unnecessary API test options as possible (including abandonedCarts, hackDeleteRangeArchivesBefore)
  - rename checkRequestResponse to assertApiResponseHasNoError