Use macro per-test-file

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

tests/cases/interpolation.2-ast.snap

@@ -0,0 +1,24 @@
+---
+source: tests/file_tests.rs
+expression: ast
+info:
+  input: "Hello! I'm {{= name }}"
+  context:
+    name: Hemera
+---
+TemplateAst {
+    root: [
+        StaticContent(
+            [Content]"Hello! I'm" (0..10),
+        ),
+        Interpolation {
+            prev_whitespace_content: Some(
+                [Whitespace]" " (10..11),
+            ),
+            expression: VariableAccess(
+                [Ident]"name" (15..19),
+            ),
+            post_whitespace_content: None,
+        },
+    ],
+}