[ticket/11843] The twig lexer fixes DEFINE variables with underscores again

https://github.com/phpbb/phpbb3/pull/1708 accidentally stopped the twig lexer
from fixing DEFINE variables with underscores in them. This commit restores
that functionality.

PHPBB3-11843
[ticket/11843] Add checking DEFINE variables with underscores to template_test

To prevent the underscore in DEFINE variables from being forgotten again,
the template tests now test INCLUDEing a file based on the contents of a
DEFINE variable with underscores in it.

PHPBB3-11843
[ticket/11843] Added newlines and included numbers in the DEFINE vars test

Per suggestion of nickvergessen, I added newlines in the DEFINE variables with
underscores test. Now if the test fails, it's easier to spot which part is
failing. Also added a test for DEFINE variables containing numbers.

PHPBB3-11843
Merge pull request #1709 from rechosen/ticket/11843

[ticket/11843] Make the twig lexer fix DEFINE variables with underscores again