Use macro per-test-file

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

tests/cases/3-instructions@trim_whitespace.snap

@@ -1,54 +0,0 @@
----
-source: tests/file_tests.rs
-expression: emit
-input_file: tests/cases/trim_whitespace.nomo
----
-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,
-    ],
-}