Add new acceptance test scenario: 034

```
  Error:
    × Main thread panicked.
    ├─▶ at /Users/ktorz/Documents/Projects/aiken-lang/aiken/crates/aiken-project/src/lib.rs:670:22
    ╰─▶ called `Result::unwrap()` on an `Err` value: FreeUnique(Name { text: "aiken/list_foldr_list_data_data_list_data", unique: Unique(1) })
  ```

examples/acceptance_tests/034/lib/tests.ak

@@ -0,0 +1,18 @@
+pub fn foldr(self: List<a>, with: fn(a, b) -> b, zero: b) -> b {
+  when self is {
+    [] -> zero
+    [x, ..xs] -> with(x, foldr(xs, with, zero))
+  }
+}
+
+pub fn flat_map(self: List<a>, with: fn(a) -> List<b>) -> List<b> {
+  foldr(self, fn(x, xs) { concat(with(x), xs) }, [])
+}
+
+test flat_map_1() {
+  flat_map([], fn(a) { [a] }) == []
+}
+
+test flat_map_2() {
+  flat_map([1, 2, 3], fn(a) { [a, a] }) == [1, 1, 2, 2, 3, 3]
+}