fix: rename test module to tests

examples/acceptance_tests/004/lib/tests.ak

@@ -0,0 +1,18 @@
+pub fn foldr(xs: List<a>, f: fn(a, b) -> b, zero: b) -> b {
+  when xs is {
+    [] -> zero
+    [x, ..rest] -> f(x, foldr(rest, f, zero))
+  }
+}
+
+pub fn prepend(x: a, xs: List<a>) -> List<a> {
+  [x, ..xs]
+}
+
+pub fn concat(left: List<a>, right: List<a>) -> List<a> {
+  foldr(left, prepend, right)
+}
+
+test concat_1() {
+  concat([1, 2, 3], [4, 5, 6]) == [1, 2, 3, 4, 5, 6]
+}