Use macro per-test-file

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

tests/cases/interpolation.3-instructions.snap

@@ -0,0 +1,30 @@
+---
+source: tests/file_tests.rs
+expression: emit
+info:
+  input: "Hello! I'm {{= name }}"
+  context:
+    name: Hemera
+---
+VMInstructions {
+    labels: {},
+    instructions: [
+        AppendContent {
+            content: "Hello! I'm" (0..10),
+        },
+        AppendContent {
+            content: " " (10..11),
+        },
+        LoadFromContextToSlot {
+            name: "name" (15..19),
+            slot: VariableSlot {
+                index: 0,
+            },
+        },
+        EmitFromSlot {
+            slot: VariableSlot {
+                index: 0,
+            },
+        },
+    ],
+}