Update the testing documentation

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.

Open the testing documentation page and locate the two quoted passages. Replace the inaccurate terminology and update the testing options to distinguish Python unit tests (-O unittest) from unit and workflow end-to-end tests (-O test); done means both sections match the wording in the issue.

Written by the indexing model from the issue text.