[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.

PHPBB3-11323
Merge PR #1194 branch 'marc1706/ticket/11323' into develop-olympus

# By Marc Alexander
# Via Marc Alexander
* marc1706/ticket/11323:
  [ticket/11323] Add tests for inclusion of defined variables