Use default rust directory structure for tests

tests/unit.rs

@@ -0,0 +1,36 @@
+extern crate rhai;
+
+use rhai::Engine;
+
+#[test]
+fn test_unit() {
+    let mut engine = Engine::new();
+
+    if let Ok(result) = engine.eval::<()>("let x = (); x") {
+        assert_eq!(result, ());
+    } else {
+        assert!(false);
+    }
+}
+
+#[test]
+fn test_unit_eq() {
+    let mut engine = Engine::new();
+
+    if let Ok(result) = engine.eval::<bool>("let x = (); let y = (); x == y") {
+        assert!(result);
+    } else {
+        assert!(false);
+    }
+}
+
+#[test]
+fn test_unit_with_spaces() {
+    let mut engine = Engine::new();
+
+    if let Ok(result) = engine.eval::<()>("let x = ( ); x") {
+        assert_eq!(result, ());
+    } else {
+        assert!(false);
+    }
+}