Simplify newline handling in libpq TAP test

Simplify newline handling in two TAP tests

pg_checksums: Use new routine to retrieve data of PG_VERSION

pg_createsubscriber: Use new routine to retrieve data of PG_VERSION

pg_resetwal: Use new routine to retrieve data of PG_VERSION

pg_combinebackup: Use new routine to retrieve data of PG_VERSION

pg_upgrade: Use new routine to retrieve data of PG_VERSION