Fix pg_rewind regression tests in VPATH builds
Fix pg_rewind regression tests in VPATH builds

Should call just "pg_rewind", instead of "./pg_rewind". The tests are called
so that PATH contains the temporariy installation bin dir.

Per report from Alvaro Herrera

oberstet/postgres-xl,pavanvd/postgres-xl,ashwinstar/gpdb,oberstet/postgres-xl,zeroae/postgres-xl,jmcatamney/gpdb,greenplum-db/gpdb,ashwinstar/gpdb,adam8157/gpdb,pavanvd/postgres-xl,ashwinstar/gpdb,zeroae/postgres-xl,50wu/gpdb,ovr/postgres-xl,zeroae/postgres-xl,zeroae/postgres-xl,techdragon/Postgres-XL,ashwinstar/gpdb,xinzweb/gpdb,oberstet/postgres-xl,ashwinstar/gpdb,xinzweb/gpdb,adam8157/gpdb,adam8157/gpdb,jmcatamney/gpdb,techdragon/Postgres-XL,50wu/gpdb,jmcatamney/gpdb,xinzweb/gpdb,lisakowen/gpdb,yazun/postgres-xl,oberstet/postgres-xl,50wu/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,Postgres-XL/Postgres-XL,greenplum-db/gpdb,50wu/gpdb,greenplum-db/gpdb,techdragon/Postgres-XL,lisakowen/gpdb,pavanvd/postgres-xl,lisakowen/gpdb,pavanvd/postgres-xl,zeroae/postgres-xl,ashwinstar/gpdb,pavanvd/postgres-xl,yazun/postgres-xl,oberstet/postgres-xl,adam8157/gpdb,Postgres-XL/Postgres-XL,greenplum-db/gpdb,50wu/gpdb,lisakowen/gpdb,ovr/postgres-xl,Postgres-XL/Postgres-XL,lisakowen/gpdb,xinzweb/gpdb,50wu/gpdb,xinzweb/gpdb,xinzweb/gpdb,jmcatamney/gpdb,jmcatamney/gpdb,ovr/postgres-xl,adam8157/gpdb,jmcatamney/gpdb,yazun/postgres-xl,50wu/gpdb,greenplum-db/gpdb,Postgres-XL/Postgres-XL,ovr/postgres-xl,xinzweb/gpdb,lisakowen/gpdb,xinzweb/gpdb,yazun/postgres-xl,Postgres-XL/Postgres-XL,lisakowen/gpdb,50wu/gpdb,ovr/postgres-xl,greenplum-db/gpdb,techdragon/Postgres-XL,lisakowen/gpdb,ashwinstar/gpdb,jmcatamney/gpdb,adam8157/gpdb,techdragon/Postgres-XL,ashwinstar/gpdb,adam8157/gpdb,adam8157/gpdb,greenplum-db/gpdb,yazun/postgres-xl