pgsql: 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

Branch
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REL_14_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/4642c2b56a99a2bd86111f8c46f377e2fc362463

Modified Files
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src/test/modules/unsafe_tests/Makefile | 3 +++
src/test/modules/unsafe_tests/README   | 2 +-
2 files changed, 4 insertions(+), 1 deletion(-)