We had forgotten to add support for this.

Fix: re-use the same logic as for items of merged views (they wrap
expressions too).

Testing: tests for the bug are added. An existing test was wrong, it
is corrected: in group_by_fd.inc, the line
--error ER_WRONG_FIELD_WITH_GROUP
before this query
 SELECT v2.pk, v2.coa
  FROM t1 LEFT JOIN v2 AS v2 ON 0
  GROUP BY v2.pk;
is removed, because the SELECT is actually valid and must
not send any error; it is made valid by this code change
in aggregate_check.cc :
- (!select->outer_join || item->type() == Item::FIELD_ITEM))
).