[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

tests/template/templates/include_define_variable.html

@@ -1,4 +1,8 @@
 <!-- DEFINE $DEF = '{VARIABLE}' -->
 <!-- INCLUDE {$DEF} -->
+
 <!-- DEFINE $DEF_WITH_UNDERSCORES = '{VARIABLE}' -->
 <!-- INCLUDE {$DEF_WITH_UNDERSCORES} -->
+
+<!-- DEFINE $DEF123 = '{VARIABLE}' -->
+<!-- INCLUDE {$DEF123} -->