Update tests/diff.rs

Co-authored-by: Alexander Theißen <alex.theissen@me.com>

tests/diff.rs

@@ -66,7 +66,7 @@ fn run_diff_test<F: FnOnce(&[u8]) -> Vec<u8>>(test_dir: &str, name: &str, test:
 			}
 		}
 
-		if std::env::var("BLESS").is_ok() {
+		if std::env::var_os("BLESS").is_some() {
 			dump(&expected_path, actual_wat.as_bytes()).expect("Failed to write to expected");
 		} else {
 			panic!();