[TASK] Add test coverage for EmailFinisher TemplatePaths

In preparation for a refactoring, test coverage is added to
the existing path merging code.

Resolves: #104862
Releases: main
Change-Id: I9f92108d565b01e3177341ef7cd0b3f25529bf91
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/85928


Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Simon Praetorius <simon@praetorius.me>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Simon Praetorius <simon@praetorius.me>