Pin standard_conforming_strings on the PostgreSQL CI test database

Pin standard_conforming_strings on the PostgreSQL CI test database

PQescapeStringConn doubles backslashes when the connection's
std_strings flag is false, which trips the backslash assertion in
src/tests/modules/sql_postgresql/escape.unlang on test servers where
the parameter isn't being reported as on. Pin it at the database
level in postgresql-setup.sh so every connection inherits it, and
restore the backslash assertion in the test.