Use macro per-test-file

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

tests/cases/trim_whitespace.3-instructions.snap

@@ -0,0 +1,58 @@
+---
+source: tests/file_tests.rs
+expression: emit
+info:
+  input: "{{ if test -}}\n    Hello {{= stuff -}}\n{{- end }}"
+  context:
+    stuff: Hemera
+    test: true
+---
+VMInstructions {
+    labels: {
+        LabelSlot {
+            index: 0,
+        }: 7,
+        LabelSlot {
+            index: 2,
+        }: 7,
+    },
+    instructions: [
+        LoadFromContextToSlot {
+            name: "test" (6..10),
+            slot: VariableSlot {
+                index: 1,
+            },
+        },
+        JumpIfNotTrue {
+            emit_slot: VariableSlot {
+                index: 1,
+            },
+            jump: LabelSlot {
+                index: 2,
+            },
+        },
+        AppendContent {
+            content: "Hello" (19..24),
+        },
+        AppendContent {
+            content: " " (24..25),
+        },
+        LoadFromContextToSlot {
+            name: "stuff" (29..34),
+            slot: VariableSlot {
+                index: 3,
+            },
+        },
+        EmitFromSlot {
+            slot: VariableSlot {
+                index: 3,
+            },
+        },
+        Jump {
+            jump: LabelSlot {
+                index: 0,
+            },
+        },
+        NoOp,
+    ],
+}