Fix SetStmt.toSql().

It needs to handle the "SET" case.  Also, add some missing test cases
for "SET".  Also, cleanup test_set/set.test.

Change-Id: I34f6005ef17e196d94366e5301251a2987746fbf
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/3620
Reviewed-by: Daniel Hecht <dhecht@cloudera.com>
Tested-by: jenkins