Use default rust directory structure for tests

tests/unary_minus.rs

@@ -0,0 +1,14 @@
+extern crate rhai;
+
+use rhai::Engine;
+
+#[test]
+fn test_unary_minus() {
+	let mut engine = Engine::new();
+
+	assert_eq!(engine.eval::<i64>("let x = -5; x").unwrap(), -5);
+
+	assert_eq!(engine.eval::<i64>("fn n(x) { -x } n(5)").unwrap(), -5);
+
+	assert_eq!(engine.eval::<i64>("5 - -(-5)").unwrap(), 0);
+}