issue #1095: Add namespace to required test files

[ScottVerbeek]

This pull requests:

• renames one test file to the correct name so that tests get executed; and
• adds qtype_stack namespace to three files, resolving fatal error in unit tests such as

  Fatal error: Cannot declare class editform_test, because the name is already in use in question/type/stack/tests/editform_test.php on line 32

Closes #1095

[sangwinc]

Thanks for fixing this - it's been a warning for a while and this looks like it's changed to a failure. Much appreciated. Chris