cleanup(test): sync test generators return String (not Future<String>)

Author: kevmoo

source_gen/test/builder_test.dart

@@ -265,14 +265,14 @@ class _LiteralGenerator extends Generator {
   const _LiteralGenerator([this._content]);
 
   @override
-  FutureOr<String> generate(_, __) => _content;
+  String generate(_, __) => _content;
 }
 
 class _BadOutputGenerator extends Generator {
   const _BadOutputGenerator();
 
   @override
-  Future<String> generate(LibraryReader library, _) async => 'not valid code!';
+  String generate(_, __) => 'not valid code!';
 }
 
 final _customHeader = '// Copyright 1979';

source_gen/test/src/unformatted_code_generator.dart

@@ -2,16 +2,14 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-import 'dart:async';
-
 import 'package:source_gen/source_gen.dart';
 
 /// Generates a single-line of unformatted code.
 class UnformattedCodeGenerator extends Generator {
   const UnformattedCodeGenerator();
 
   @override
-  Future<String> generate(LibraryReader library, _) async => unformattedCode;
+  String generate(_, __) => unformattedCode;
 
   static const formattedCode = '''
 void hello() => print('hello');