fix: rename test module to tests

examples/acceptance_tests/020/lib/tests.ak

@@ -0,0 +1,10 @@
+pub fn map(opt: Option<a>, f: fn(a) -> result) -> Option<result> {
+  when opt is {
+    None -> None
+    Some(a) -> Some(f(a))
+  }
+}
+
+test map_2() {
+  map(Some(14), fn(n) { n + 1 }) == Some(15)
+}