fix: rename test module to tests

examples/acceptance_tests/005/lib/tests.ak

@@ -0,0 +1,16 @@
+use aiken/builtin.{head_list}
+
+pub fn head(xs: List<a>) -> Option<a> {
+  when xs is {
+    [] -> None
+    _ -> Some(head_list(xs))
+  }
+}
+
+test head_1() {
+  head([1, 2, 3]) == Some(1)
+}
+
+test head_2() {
+  head([]) == None
+}