In mtr.check_warnings, `text` was declares as type text, which is
64K, and when the server log grows larger than this, it would be
truncated, and then check_warnings was actually checking the 
error messages of a previous test and complain warnings.

This patch fixed the problem by change the type of `text` to
mediumtext, which is 16M.

mysql-test/include/mtr_warnings.sql:
  change the type of `text` to mediumtext