[ticket/12009] Do not allow incorrectly structured extensions to be installed.

PHPBB3-12009
[ticket/12009] Move valid extensions in tests to correct dir structure.

PHPBB3-12009
[ticket/12009] Fix functional tests.

PHPBB3-12009
[ticket/12009] Update functional test to check for invalid extensions.

PHPBB3-12009
[ticket/12009] Prevent user from enabling invalid extension through direct URL

PHPBB3-12009
Merge remote-tracking branch 'prototech/ticket/12009' into develop

* prototech/ticket/12009:
  [ticket/12009] Prevent user from enabling invalid extension through direct URL
  [ticket/12009] Update functional test to check for invalid extensions.
  [ticket/12009] Fix functional tests.
  [ticket/12009] Move valid extensions in tests to correct dir structure.
  [ticket/12009] Do not allow incorrectly structured extensions to be installed.