Add new acceptance test scenario (012)

```
  Error:
    × Main thread panicked.
    ├─▶ at crates/lang/src/uplc.rs:3413:34
    ╰─▶ internal error: entered unreachable code: Var {
            scope: [
                0,
                1,
                3,
                24,
                25,
                29,
                32,
            ],
            constructor: ValueConstructor {
                public: false,
                variant: LocalVariable {
                    location: 46..62,
                },
                tipo: Fn {
                    args: [
                        Var {
                            tipo: RefCell {
                                value: Link {
                                    tipo: App {
                                        public: true,
                                        module: "",
                                        name: "Int",
                                        args: [],
                                    },
                                },
                            },
                        },
                    ],
                    ret: App {
                        public: true,
                        module: "",
                        name: "Bool",
                        args: [],
                    },
                },
            },
            name: "f",
            variant_name: "",
        }
  ```

examples/acceptance_tests/012/aiken.toml

@@ -0,0 +1,2 @@
+name = "acceptance_test_012"
+version = "0.0.0"

examples/acceptance_tests/012/lib/test.ak

@@ -0,0 +1,18 @@
+use aiken/builtin
+
+pub fn filter(xs: List<a>, f: fn(a) -> Bool) -> List<a> {
+  when xs is {
+    [] -> []
+    [x, ..rest] ->
+      if f(x) {
+        [x, ..filter(rest, f)]
+      } else {
+        filter(rest, f)
+      }
+  }
+}
+
+test filter_1() {
+  filter([1,
+      2, 3, 4, 5, 6], fn(x) { builtin.mod_integer(x, 2) == 0 }) == [2, 4, 6]
+}