Fix gen_defaults test randomness (#142743)

This PR improves the gen_defaults tests to not be tied to a particular order of execution.
Since there is a global class that holds the state of the used/not used tokens, we need to clear this logger before each test.

Fixes https://github.com/flutter/flutter/issues/142716

cc @zanderso @QuncCccccc 

*If you had to change anything in the [flutter/tests] repo, include a link to the migration guide as per the [breaking change policy].*

dev/tools/gen_defaults/test/gen_defaults_test.dart

@@ -19,8 +19,16 @@ import 'package:test/test.dart';
 
 void main() {
   final TokenLogger logger = tokenLogger;
+  // Required init with empty at least once to init late fields.
+  // Then we can use the `clear` method.
   logger.init(allTokens: <String, dynamic>{}, versionMap: <String, List<String>>{});
 
+  setUp(() {
+    // Cleanup the global token logger before each test, to not be tied to a particular
+    // test order.
+    logger.clear();
+  });
+
   test('Templates will append to the end of a file', () {
     final Directory tempDir = Directory.systemTemp.createTempSync('gen_defaults');
     try {