Add tests for for loop

Signed-off-by: Marcel Müller <neikos@neikos.email>

tests/cases/4-output@simple_for.snap

@@ -2,10 +2,12 @@
 source: tests/file_tests.rs
 expression: output
 info:
+  input: "{{ for value in values -}}\n    {{-= value }}\n{{- end }}\n{{ for value in no_values -}}\n    {{-= value }}\n{{ else -}}\nNo Values >:C\n{{- end }}"
   context:
+    no_values: []
     values:
       - one
       - two
 input_file: tests/cases/simple_for.nomo
 ---
-"one\ntwo\n"
+"one\ntwo\n\nNo Values >:C"