Only run dart files as a test in the Android preview tools test shard…

… (#140099)

I added a README at the end of flutter/flutter#131901 and did not realize that it was being run as a test, [leading to test failures (of course)](https://logs.chromium.org/logs/flutter/buildbucket/cr-buildbucket/8761789207346423409/+/u/run_test.dart_for_android_preview_tool_integration_tests_shard_and_subshard_None/test_stdout). This makes it so we only run dart files as dart tests.

dev/bots/test.dart

@@ -482,7 +482,7 @@ Future<void> _runAndroidPreviewIntegrationToolTests() async {
   final List<String> allTests = Directory(path.join(_toolsPath, 'test', 'android_preview_integration.shard'))
       .listSync(recursive: true).whereType<File>()
       .map<String>((FileSystemEntity entry) => path.relative(entry.path, from: _toolsPath))
-      .toList();
+      .where((String testPath) => path.basename(testPath).endsWith('_test.dart')).toList();
 
   await _runDartTest(
     _toolsPath,