MDL-69451 dml: use same temptables for both rw and ro database handle

moodle_read_slave_trait: when creating another handle, restore temptables
property that is clobbered by raw_connect().

Also a better condition for temptable related queries detection in
pgsql_native_moodle_database.

dml_pgsql_read_slave_test::test_temp_table(): use real db connection
if possible, otherwise skip the test.

lib/dml/pgsql_native_moodle_database.php

@@ -295,7 +295,7 @@ class pgsql_native_moodle_database extends moodle_database {
         }
 
         // ... a nuisance - temptables use this.
-        if (preg_match('/\bpg_constraint/', $sql) && $this->temptables->get_temptables()) {
+        if (preg_match('/\bpg_catalog/', $sql) && $this->temptables->get_temptables()) {
             return false;
         }