Add new acceptance test scenario 028

```
  Error:
    × Main thread panicked.
    ├─▶ at crates/lang/src/builder.rs:1257:30
    ╰─▶ internal error: entered unreachable code
  ```

examples/acceptance_tests/028/aiken.toml

@@ -0,0 +1,2 @@
+name = "acceptance_test_028"
+version = "0.0.0"

examples/acceptance_tests/028/lib/test.ak

@@ -0,0 +1,27 @@
+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)
+      }
+  }
+}
+
+pub fn unique(xs: List<a>) -> List<a> {
+  when xs is {
+    [] -> []
+    [x, ..rest] -> [x, ..unique(filter(rest, fn(y) { y != x }))]
+  }
+}
+
+test unique_1() {
+  unique([]) == []
+}
+
+test unique_2() {
+  let xs = [1, 2, 3, 1]
+  unique(xs) == [1, 2, 3]
+}