One test was relying on method remove_tree that isn't implemented in the
oldest Perl we support; fix it by using the older rmtree instead.

Another test had a typo in a SQL command, which isn't noticed because
the PostgresNode->psql() method doesn't check that queries return
correctly.  That's undesirable and will also be fixed later on, but for
now let's make the test actually work.

Author: Craig Ringer