Refactor tests.

tests/increment.rs

@@ -1,12 +1,14 @@
-use rhai::Engine;
+use rhai::{Engine, EvalAltResult};
 
 #[test]
-fn test_increment() {
+fn test_increment() -> Result<(), EvalAltResult> {
     let mut engine = Engine::new();
 
-    assert_eq!(engine.eval::<i64>("let x = 1; x += 2; x"), Ok(3));
+    assert_eq!(engine.eval::<i64>("let x = 1; x += 2; x")?, 3);
     assert_eq!(
-        engine.eval::<String>("let s = \"test\"; s += \"ing\"; s"),
-        Ok("testing".to_string())
+        engine.eval::<String>("let s = \"test\"; s += \"ing\"; s")?,
+        "testing".to_string()
     );
+
+    Ok(())
 }