Review for subsequent C++ accessibility test infrastructure

[Colomban Wendling]

Hello everyone!

I'm again looking for review on a few changests related to CppUnit a11y 
tests:
- https://gerrit.libreoffice.org/c/core/+/137307
- https://gerrit.libreoffice.org/c/core/+/137335
- https://gerrit.libreoffice.org/c/core/+/137336
- https://gerrit.libreoffice.org/c/core/+/137337

All changests depend on the first one that moves things around for use 
more widely, and the first and last are the bigger ones (third is trivial).

I have a bizarre test failure on Windows with the last one, and I'm not 
sure what to do about it, see 
https://gerrit.libreoffice.org/c/core/+/137337/comments/7f98e83b_4de322ee 
-- any advice would be much appreciated.

I'm not sure whether I should add reviewers myself based on the ones 
from the previous submissions in that area or if that'd be a tad 
inconsiderate.

Thanks in advance,
Colomban