Refactor tests.

tests/string.rs

@@ -1,15 +1,17 @@
-use rhai::Engine;
+use rhai::{Engine, EvalAltResult};
 
 #[test]
-fn test_string() {
+fn test_string() -> Result<(), EvalAltResult> {
     let mut engine = Engine::new();
 
     assert_eq!(
-        engine.eval::<String>("\"Test string: \\u2764\""),
-        Ok("Test string: ❤".to_string())
+        engine.eval::<String>("\"Test string: \\u2764\"")?,
+        "Test string: ❤".to_string()
     );
     assert_eq!(
-        engine.eval::<String>("\"foo\" + \"bar\""),
-        Ok("foobar".to_string())
+        engine.eval::<String>("\"foo\" + \"bar\"")?,
+        "foobar".to_string()
     );
+
+    Ok(())
 }