fix: rename test module to tests

examples/acceptance_tests/012/lib/tests.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]
+}