Added a test case for bug #8614.
sql_select.cc:
  Fixed bug #8614.
  SELECT DISTINCT ... GROUP BY 'const' must be equivalent to
  SELECT ... GROUP BY 'const'.


sql/sql_select.cc:
  Fixed bug #8614.
  SELECT DISTINCT ... GROUP BY 'const' must be equivalent to
  SELECT ... GROUP BY 'const'.
mysql-test/t/group_by.test:
  Added a test case for bug #8614.
mysql-test/r/group_by.result:
  Added a test case for bug #8614.