Mark unsafe_tests module as not runnable with installcheck

Mark unsafe_tests module as not runnable with installcheck

This was an omission in the original creation of the module.

Also slightly adjust some wording to avoid a double "is".

Backpatch the non-meson piece of this to release 12, where the module
was introduced.

Discussion: https://postgr.es/m/be869e1c-8e3f-4cde-8609-212c899cccf9@dunslane.net