- Added more tests to the MWL#89 specific test, and made the test more modular.
- Updated test files.
- Fixed a memory leak.
- More comments.

mysql-test/r/subselect_mat.result:
  - Updated the test file to reflect the new optimizer switches related to
    materialized subquery execution.
  - Added one extra test to test all cases that expose BUG#40037 (this is an old bug from 5.x).
  - Updated the test result with correct results that expose BUG#40037.
mysql-test/t/subselect_mat.test:
  - Updated the test file to reflect the new optimizer switches related to
    materialized subquery execution.
  - Added one extra test to test all cases that expose BUG#40037 (this is an old bug from 5.x).
  - Updated the test result with correct results that expose BUG#40037.
sql/sql_select.cc:
  Fixed a memory leak reported by Valgrind.