Use default rust directory structure for tests

tests/decrement.rs

@@ -0,0 +1,20 @@
+extern crate rhai;
+
+use rhai::Engine;
+
+#[test]
+fn test_decrement() {
+    let mut engine = Engine::new();
+
+    if let Ok(result) = engine.eval::<i64>("let x = 10; x -= 7; x") {
+        assert_eq!(result, 3);
+    } else {
+        assert!(false);
+    }
+
+    if let Ok(_) = engine.eval::<String>("let s = \"test\"; s -= \"ing\"; s") {
+        assert!(false);
+    } else {
+        assert!(true);
+    }
+}