[TASK] EXT:form - add unitTests for class TypoScriptToJsonConverter

This patch introduces a first test case for TypoScriptToJsonConverter
of EXT:form. The test case asserts that a protected method internally
triggers adding elements to a given and mocked parent element in the
nested form hierarchy.

Resolves: #74337
Releases: master, 7.6
Change-Id: I191ba8fe41a5829e23303447f86303ae8c6dd081
Reviewed-on: https://review.typo3.org/47122


Reviewed-by: Oliver Hader <oliver.hader@typo3.org>
Tested-by: Oliver Hader <oliver.hader@typo3.org>
Reviewed-by: Andreas Wolf <andreas.wolf@typo3.org>
Tested-by: Andreas Wolf <andreas.wolf@typo3.org>