test: don't include output when nothing has been changed by fixers

tests/src/rule-tester.js

@@ -5,7 +5,7 @@ import semver from 'semver';
 export const usingFlatConfig = semver.major(eslintVersion) >= 9;
 
 export function withoutAutofixOutput(test) {
-  return { ...test, output: test.code };
+  return { ...test, ...usingFlatConfig || { output: test.code } };
 }
 
 export class FlatCompatRuleTester {