[ticket/11323] Add tests for inclusion of defined variables

This adds 2 tests for the template engine. The test using
include_define_variable.html will test if a defined variable, which was
defined with another template variable, can be used to include a file.
The second test will do the same inside a loop using a loop variable.

Ticket: http://tracker.phpbb.com/browse/PHPBB3-11323

PHPBB3-11323