Use parallel installcheck in vcregress.pl's upgrade test
Use parallel installcheck in vcregress.pl's upgrade test

This is to keep the test in sync with what's done in test.sh, which
acquired this change in commit da906766c.

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