Use macro per-test-file

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

tests/cases/simple_for.4-output.snap

@@ -0,0 +1,12 @@
+---
+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
+---
+"one\ntwo\n\nNo Values >:C"