Use macro per-test-file

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

tests/cases/if_else_if.4-output.snap

@@ -0,0 +1,11 @@
+---
+source: tests/file_tests.rs
+expression: output
+info:
+  input: "{{ if test }}\n    Not Hello World! :C\n{{ else if another_test }}\n    Hello World!\n{{ end }}"
+  context:
+    test: false
+    stuff: more
+    another_test: true
+---
+"\n    Hello World!\n"