Use default rust directory structure for tests

tests/mismatched_op.rs

@@ -0,0 +1,13 @@
+extern crate rhai;
+
+use rhai::{Engine, EvalAltResult};
+
+#[test]
+fn test_mismatched_op() {
+    let mut engine = Engine::new();
+
+    assert_eq!(
+        engine.eval::<i64>("60 + \"hello\""),
+        Err(EvalAltResult::ErrorFunctionNotFound)
+    );
+}