Test: fix 001.loadbalance test failure.

Test: fix 001.loadbalance test failure.

Since this commit:
https://git.postgresql.org/gitweb/?p=pgpool2.git;a=commit;h=f8c3d7b49a1c6496eca2203e95248b86c204bbfa

001.loadbalance test fails on pre PostgreSQL 15, because of the change
in psql.  PostgreSQL 15's psql now prints the whole query results in a
multi-statement query.  Consider following example:

SELECT \BEGIN\;SELECT 1;ROLLBACK;

In 15, the result of SELECT will be printed by psql, while pre-15 will
not.  This causes some test results not to match with expected
results.  To fix this, I added expected7-r-pre15 etc. for pre-15.
test.sh now recognizes "-pre15" suffix. If the server version is
pre-15 and an expected file with suffix "pre-15" exists, the file will
be used.