[TASK] Improve AbstractDataHandlerActionTestCase

Improvements for the AbstractDataHandlerActionTestCase to
get better functional test results in general and working
functional tests in impexp (e.g. for FlexForm XML):

* collect all fail messages before calling fail()
* check records for left rows that was not assigned in csv
* do strict compare if value is NULL
* compare XML, e.g. FlexForm

Resolves: #56241
Releases: 6.2
Change-Id: Ic21c27261cbe18dc5fd37c4869c10264880b731e
Reviewed-on: https://review.typo3.org/27816
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn