Refactor tests.

tests/decrement.rs

@@ -1,16 +1,17 @@
-use rhai::Engine;
-use rhai::EvalAltResult;
+use rhai::{Engine, EvalAltResult};
 
 #[test]
-fn test_decrement() {
+fn test_decrement() -> Result<(), EvalAltResult> {
     let mut engine = Engine::new();
 
-    assert_eq!(engine.eval::<i64>("let x = 10; x -= 7; x"), Ok(3));
+    assert_eq!(engine.eval::<i64>("let x = 10; x -= 7; x")?, 3);
 
-    assert_eq!(
-        engine.eval::<String>("let s = \"test\"; s -= \"ing\"; s"),
-        Err(EvalAltResult::ErrorFunctionNotFound(
-            "- (alloc::string::String, alloc::string::String)".to_string()
-        ))
-    );
+    let r = engine.eval::<String>("let s = \"test\"; s -= \"ing\"; s");
+
+    match r {
+        Err(EvalAltResult::ErrorFunctionNotFound(err, _)) if err.starts_with("- ") => (),
+        _ => panic!(),
+    }
+
+    Ok(())
 }