[TASK] Refactor ext:impexp functional tests

The ext:impexp functional tests are rather hard to read and
follow. The patch untangles them to make them better maintainable:

* Combine the two different import / export abstract classes into 1 class
* Don't inherit from AbstractDataHandlerActionTestCase anymore
* Give single test cases better file names and do not hide them in sub dirs
* Move all fixtures to Fixtures/ dir
* Use less helper methods to import and test things
* File paths are more straight and with less helper magic

Change-Id: Iae36771b94045f99444f394129cbcf183168b584
Resolves: #81393
Releases: master, 8.7
Reviewed-on: https://review.typo3.org/52981


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>