fix: rename test module to tests

examples/acceptance_tests/009/lib/tests.ak

@@ -0,0 +1,9 @@
+use aiken/builtin.{length_of_bytearray}
+
+pub fn is_empty(bytes: ByteArray) -> Bool {
+  length_of_bytearray(bytes) == 0
+}
+
+test is_empty_1() {
+  is_empty(#[]) == True
+}