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

src/test/modules/unsafe_tests/Makefile

@@ -2,6 +2,9 @@
 
 REGRESS = rolenames alter_system_table guc_privs
 
+# the whole point of these tests is to not run installcheck
+NO_INSTALLCHECK = 1
+
 ifdef USE_PGXS
 PG_CONFIG = pg_config
 PGXS := $(shell $(PG_CONFIG) --pgxs)

src/test/modules/unsafe_tests/README

@@ -1,6 +1,6 @@
 This directory doesn't actually contain any extension module.
 
-What it is is a home for regression tests that we don't want to run
+Instead it is a home for regression tests that we don't want to run
 during "make installcheck" because they could have side-effects that
 seem undesirable for a production installation.
 

src/test/modules/unsafe_tests/meson.build

@@ -10,5 +10,6 @@ tests += {
       'alter_system_table',
       'guc_privs',
     ],
+    'runningcheck': false,
   },
 }