[TASK] Streamline DataHandler functional tests

* Obsolete importScenarioDataSet() by using
  importCSVDataSet() directly with absolute paths.
  IDE's now resolve these paths so they can be
  clicked to open, which is *much* more convenient
  when working with the .csv files.
* Similar, obsolete assertAssertionDataSet() by using
  assertCSVDataSet() directly.
* Protected constants.
* Late static binding for workspace init in setUp()
  to simplify and streamline workspace test inheritance.
* A couple of minor streamlinings.

Change-Id: I9e814335b863da278e814641f6995a24778da95b
Resolves: #96104
Releases: master, 11.5
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/72326


Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>