Use macro per-test-file

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

tests/cases/interpolation.1-parsed.snap

@@ -0,0 +1,20 @@
+---
+source: tests/file_tests.rs
+expression: parsed
+info:
+  input: "Hello! I'm {{= name }}"
+  context:
+    name: Hemera
+---
+ParsedTemplate {
+    tokens: [
+        [Content]"Hello! I'm" (0..10),
+        [Whitespace]" " (10..11),
+        [LeftDelim]"{{" (11..13),
+        [WantsOutput]"=" (13..14),
+        [Whitespace]" " (14..15),
+        [Ident]"name" (15..19),
+        [Whitespace]" " (19..20),
+        [RightDelim]"}}" (20..22),
+    ],
+}