Added test cases for bug #7894.
sql_select.cc:
  Fixed bug #7894: GROUP BY queries with ROLLUP returned
  wrong results for expressions containing group by columns.
  The fix ensured correct results by replacement of all
  occurrences of group by fields in non-aggregate expressions
  for corresponding ref objects and preventing creation of
  fields in temporary tables for expression containing group
  by fields.


sql/sql_select.cc:
  Fixed bug #7894: GROUP BY queries with ROLLUP returned
  wrong results for expressions containing group by columns.
  The fix ensured correct results by replacement of all
  occurrences of group by fields in non-aggregate expressions
  for corresponding ref objects and preventing creation of
  fields in temporary tables for expression containing group
  by fields.
mysql-test/t/olap.test:
  Added test cases for bug #7894.
mysql-test/r/olap.result:
  Added test cases for bug #7894.