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
When template tests are skipped because cache directory is not
writable, include path to the cache directory into the message
saying it is not writable.
PHPBB3-10354
Due to the usage of `__DIR__` for the file includes the tests can't
be ran on systems with PHP < 5.3. Replace all occurances of
`__DIR__` with `dirname(__FILE__)`.
PHPBB3-10011
