Fix occasional regression test 018.detach_primary error.

Fix occasional regression test 018.detach_primary error.

According to the buildfarm log, the test failed at after this:
with this:
$PGPOOL_INSTALL_DIR/bin/pcp_watchdog_info -v -w -p $PCP_PORT
error message was:
ERROR: connection to socket "/tmp/.s.PGSQL.50001" failed with error "No such file or directory"
This suggests that pcp_watchdog_info fails because pcp server has not started yet.

To fix this add wait_for_pgpool_startup before pcp_watchdog_info.