Update the testing documentation

[ehogan]

Following #552, there is some information that was originally on the testing documentation page that needs updating:

• The text "Acceptance tests which run in the compare task verify if the correct outputs are produced by the
  workflow" isn't quite correct. The tests that use the compare task are workflow end-to-end (system) tests, not acceptance tests. The text should use "the workflow end-to-end tests ensure that the expected output files are generated by CMEW" (expected rather than correct).
• The text "Run either the workflow end-to-end (acceptance) tests or Python unit tests" isn't quite correct. The options for testing are:
  • Python unit tests only (-O unittest)
  • Python unit tests and the workflow end-to-end tests (-O test)

Make these changes to the testing documentation.