Add a test on join cross PostgreSQL and HBase

com.oracle.truffle.r.test/tests/QueryJoinCrossDatabases.R

+emp = new.tableRef("emp", "PostgreSQL", "postgre.config", "public")
+dept = new.tableRef("dept", "HBase", "hbase-site.xml", "default")
+q = select(function (x) {
+             res = new.env()
+             res$ename = x$ename
+             res },
+    where(function (x) x$loc == "NEW YORK",
+    join(from(emp), from(dept), function (x,y) x$deptno == y$deptno)))
+results = query.force(q)
+print(results)

com.oracle.truffle.r.test/tests/QueryJoinCrossDatabases.out

+   ename
+1  CLARK
+2   KING
+3 MILLER
+