Refactor tests.

tests/while_loop.rs

@@ -1,14 +1,16 @@
-use rhai::Engine;
+use rhai::{Engine, EvalAltResult};
 
 #[test]
-fn test_while() {
+fn test_while() -> Result<(), EvalAltResult> {
     let mut engine = Engine::new();
 
     assert_eq!(
         engine.eval::<i64>(
             "let x = 0; while x < 10 { x = x + 1; if x > 5 { \
              break } } x",
-        ),
-        Ok(6)
+        )?,
+        6
     );
+
+    Ok(())
 }