Add a test to probe the cache similar to the email test func.



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Co-authored-by: delvh <dev.lh@web.de>
Co-authored-by: silverwind <me@silverwind.io>
(cherry picked from commit 363c1235987793dffa5cc851aaae585eb81f091e)
Conflicts:
options/locale/locale_en-US.ini
templates/admin/self_check.tmpl
trivial context conflict