Fix ROLLBACK TO command to work in aborted transaction.

Fix ROLLBACK TO command to work in aborted transaction.

Commit:

https://git.postgresql.org/gitweb/?p=pgpool2.git;a=commit;h=eea522ebfcf791a623e865deaa1aa6fb59e3c50b

fixed some cases in aborted transaction so that SQL commands except
COMMIT/ROLLBACK are not forwarded to backend. But it missed ROLLBACK
TO command. As a result even if ROLLBACK TO command is issued, the
command was not forwarded to backend and the transaction keeps on in
aborted state.
Back patched through 4.3 which the commit was brought in.
Also add a test case for ROLLBACK TO to test 078.
Discussion:
https://www.pgpool.net/pipermail/pgpool-general-jp/2022-November/001715.html