fix: rename test module to tests

examples/acceptance_tests/001/lib/tests.ak

@@ -0,0 +1,14 @@
+pub fn length(xs: List<a>) -> Int {
+  when xs is {
+    [] -> 0
+    [_, ..rest] -> 1 + length(rest)
+  }
+}
+
+test length_1() {
+  length([1, 2, 3]) == 3
+}
+
+test length_2() {
+  length([]) == 0
+}