[ticket/11579] Add basic set of unit tests for validate_data()

This currently includes tests for the string, num, date, match, and
language iso validation functions.

PHPBB3-11579
[ticket/11579] Add remaining unit tests for validate_data functions

This now includes tests for usernames, passwords, emails, and jabber
addresses. A few small changes had to be applied to phpbb_mock_cache
and phpbb_mock_user in order to incorporate needed methods.

PHPBB3-11579
[ticket/11579] Remove unnecessary globals from validate_password()

The globals $db and $user are not used in that function.

PHPBB3-11579
[ticket/11579] Move tests into seperate files depending on needed fixture

PHPBB3-11579
[ticket/11579] Use test case helper class and use assert prefix for method

PHPBB3-11579
[ticket/11579] Move simple tests into seperate files

PHPBB3-11579
[ticket/11579] Rework calls to validate_data_helper

PHPBB3-11579
[ticket/11579] Add missing commas to validate_username_test

PHPBB3-11579
[ticket/11579] Do not extend validate_data_helper

PHPBB3-11579
[ticket/11579] Add method for validating emails for valid MX and mark as slow

A method for setting up the prerequisities also has been added in order to
reduce the amount of necessary code.

PHPBB3-11579
Merge remote-tracking branch 'marc1706/ticket/11579' into develop-olympus

* marc1706/ticket/11579:
  [ticket/11579] Add method for validating emails for valid MX and mark as slow
  [ticket/11579] Do not extend validate_data_helper
  [ticket/11579] Add missing commas to validate_username_test
  [ticket/11579] Rework calls to validate_data_helper
  [ticket/11579] Move simple tests into seperate files
  [ticket/11579] Use test case helper class and use assert prefix for method
  [ticket/11579] Move tests into seperate files depending on needed fixture
  [ticket/11579] Remove unnecessary globals from validate_password()
  [ticket/11579] Add remaining unit tests for validate_data functions
  [ticket/11579] Add basic set of unit tests for validate_data()