[path_provider] De-flake getExternalStorageDirectories test (#5628)

Fixes the flaky path provider test. 

Directories.listSync will sometimes not return the files in the directory (as discussed in https://github.com/flutter/flutter/issues/139378), but the files are there and (after letting it loop for quite a while and comparing to current flake) they always appear in the output of `Process.runSync('ls', ...)`.

packages/path_provider/path_provider/example/integration_test/path_provider_test.dart

@@ -117,6 +117,13 @@ void _verifySampleFile(Directory? directory, String name) {
 
   file.writeAsStringSync('Hello world!');
   expect(file.readAsStringSync(), 'Hello world!');
-  expect(directory.listSync(), isNotEmpty);
+  // This check intentionally avoids using Directory.listSync on Android due to
+  // https://github.com/dart-lang/sdk/issues/54287.
+  if (Platform.isAndroid) {
+    expect(
+        Process.runSync('ls', <String>[directory.path]).stdout, contains(name));
+  } else {
+    expect(directory.listSync(), isNotEmpty);
+  }
   file.deleteSync();
 }

packages/path_provider/path_provider_android/example/integration_test/path_provider_test.dart

@@ -101,6 +101,9 @@ void _verifySampleFile(String? directoryPath, String name) {
 
   file.writeAsStringSync('Hello world!');
   expect(file.readAsStringSync(), 'Hello world!');
-  expect(directory.listSync(), isNotEmpty);
+  // This check intentionally avoids using Directory.listSync due to
+  // https://github.com/dart-lang/sdk/issues/54287.
+  expect(
+      Process.runSync('ls', <String>[directory.path]).stdout, contains(name));
   file.deleteSync();
 }