[TASK] Improve Test

Refine the test for DateTimeConverter timezone handling.

The suggested backport from Flow Fluid was already in place, but the
improved test was missing.

Resolves: #44213
Releases: 6.1
Change-Id: I3c7d73c1bba8ebecc5d49020d155d5d27c256961
Reviewed-on: https://review.typo3.org/18155
Reviewed-by: Markus Günther
Tested-by: Markus Günther
Reviewed-by: Daniel Hürtgen
Reviewed-by: Wouter Wolters
Reviewed-by: Marc Bastian Heinrichs
Tested-by: Marc Bastian Heinrichs