gpconfig: improve test performance
gpconfig: improve test performance

Change to `gpstop -u` for a quick GUC reload as opposed to a full
restart during tests. This requires us to choose GUCs that can be
reloaded during a SIGHUP (e.g. change gp_resource_group_cpu_limit to
checkpoint_completion_target).

Co-authored-by: Mark Sliva <4a8deaf7c198187905290eb5e4e04145ada902a2@pivotal.io>

50wu/gpdb,lisakowen/gpdb,greenplum-db/gpdb,50wu/gpdb,greenplum-db/gpdb,lisakowen/gpdb,jmcatamney/gpdb,50wu/gpdb,ashwinstar/gpdb,lisakowen/gpdb,adam8157/gpdb,adam8157/gpdb,lisakowen/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,50wu/gpdb,adam8157/gpdb,ashwinstar/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,ashwinstar/gpdb,jmcatamney/gpdb,xinzweb/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,50wu/gpdb,xinzweb/gpdb,xinzweb/gpdb,xinzweb/gpdb,adam8157/gpdb,lisakowen/gpdb,50wu/gpdb,adam8157/gpdb,ashwinstar/gpdb,lisakowen/gpdb,jmcatamney/gpdb,50wu/gpdb,jmcatamney/gpdb,50wu/gpdb,jmcatamney/gpdb,adam8157/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,xinzweb/gpdb,xinzweb/gpdb,ashwinstar/gpdb,lisakowen/gpdb,greenplum-db/gpdb,ashwinstar/gpdb,lisakowen/gpdb,greenplum-db/gpdb,adam8157/gpdb,ashwinstar/gpdb,xinzweb/gpdb,adam8157/gpdb,xinzweb/gpdb