fix: rename test module to tests

examples/acceptance_tests/018/lib/tests.ak

@@ -0,0 +1,10 @@
+pub fn or_else(opt: Option<a>, default: a) -> a {
+  when opt is {
+    None -> default
+    Some(a) -> a
+  }
+}
+
+test or_else_2() {
+  or_else(Some(42), 14) == 42
+}