No more string matching


mysql-test/include/check-warnings.test:
  Check return value from check_warnings and call skip if ok
mysql-test/include/mtr_check.sql:
  Improve readability
mysql-test/include/mtr_warnings.sql:
  Add out parameter for result.
  Increase the max_allowed_packet variable so the load_file works
  even if the server is started with low such value