Use default rust directory structure for tests

tests/string.rs

@@ -0,0 +1,20 @@
+extern crate rhai;
+
+use rhai::Engine;
+
+#[test]
+fn test_string() {
+    let mut engine = Engine::new();
+
+    if let Ok(result) = engine.eval::<String>("\"Test string: \\u2764\"") {
+        assert_eq!(result, "Test string: ❤");
+    } else {
+        assert!(false);
+    }
+
+    if let Ok(result) = engine.eval::<String>("\"foo\" + \"bar\"") {
+        assert_eq!(result, "foobar");
+    } else {
+        assert!(false);
+    }
+}