Re-add "make check" target in src/test/isolation/Makefile
Re-add "make check" target in src/test/isolation/Makefile

This effectively reverts 7886cc73ad12fb9b5a729b6c8152f11a309f5d65,
which was done under the impression that isolationtester needs libpq,
which it no longer does (and never really did).

snaga/postgres-xl,adam8157/gpdb,arcivanov/postgres-xl,50wu/gpdb,greenplum-db/gpdb,tpostgres-projects/tPostgres,adam8157/gpdb,jmcatamney/gpdb,ovr/postgres-xl,xinzweb/gpdb,zeroae/postgres-xl,xinzweb/gpdb,zeroae/postgres-xl,ashwinstar/gpdb,lisakowen/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,zeroae/postgres-xl,greenplum-db/gpdb,zeroae/postgres-xl,adam8157/gpdb,jmcatamney/gpdb,jmcatamney/gpdb,snaga/postgres-xl,arcivanov/postgres-xl,lisakowen/gpdb,arcivanov/postgres-xl,ashwinstar/gpdb,pavanvd/postgres-xl,techdragon/Postgres-XL,postmind-net/postgres-xl,yazun/postgres-xl,snaga/postgres-xl,ashwinstar/gpdb,greenplum-db/gpdb,Postgres-XL/Postgres-XL,kmjungersen/PostgresXL,xinzweb/gpdb,lisakowen/gpdb,yazun/postgres-xl,oberstet/postgres-xl,oberstet/postgres-xl,xinzweb/gpdb,jmcatamney/gpdb,Postgres-XL/Postgres-XL,arcivanov/postgres-xl,adam8157/gpdb,yazun/postgres-xl,Postgres-XL/Postgres-XL,ashwinstar/gpdb,snaga/postgres-xl,lisakowen/gpdb,lisakowen/gpdb,ovr/postgres-xl,tpostgres-projects/tPostgres,greenplum-db/gpdb,lisakowen/gpdb,Postgres-XL/Postgres-XL,jmcatamney/gpdb,ovr/postgres-xl,50wu/gpdb,pavanvd/postgres-xl,50wu/gpdb,adam8157/gpdb,jmcatamney/gpdb,pavanvd/postgres-xl,tpostgres-projects/tPostgres,oberstet/postgres-xl,xinzweb/gpdb,50wu/gpdb,techdragon/Postgres-XL,50wu/gpdb,arcivanov/postgres-xl,greenplum-db/gpdb,kmjungersen/PostgresXL,arcivanov/postgres-xl,pavanvd/postgres-xl,adam8157/gpdb,ashwinstar/gpdb,xinzweb/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,Postgres-XL/Postgres-XL,oberstet/postgres-xl,zeroae/postgres-xl,snaga/postgres-xl,adam8157/gpdb,jmcatamney/gpdb,tpostgres-projects/tPostgres,xinzweb/gpdb,yazun/postgres-xl,50wu/gpdb,postmind-net/postgres-xl,adam8157/gpdb,kmjungersen/PostgresXL,postmind-net/postgres-xl,pavanvd/postgres-xl,techdragon/Postgres-XL,lisakowen/gpdb,oberstet/postgres-xl,ovr/postgres-xl,50wu/gpdb,xinzweb/gpdb,techdragon/Postgres-XL,postmind-net/postgres-xl,50wu/gpdb,greenplum-db/gpdb,lisakowen/gpdb,kmjungersen/PostgresXL,tpostgres-projects/tPostgres,ashwinstar/gpdb,ovr/postgres-xl,yazun/postgres-xl,techdragon/Postgres-XL,kmjungersen/PostgresXL,postmind-net/postgres-xl