Mac plugin_test_macos is 2.04% flaky

The post-submit test builder `Mac plugin_test_macos` had a flaky ratio 2.04% for the past (up to) 100 commits, which is above our 2.00% threshold.

One recent flaky example for a same commit: https://ci.chromium.org/ui/p/flutter/builders/prod/Mac%20plugin_test_macos/20903
Commit: https://github.com/flutter/flutter/commit/435cf9a408967a716ef9828e83af03dae289b658

Flaky builds:
https://ci.chromium.org/ui/p/flutter/builders/prod/Mac%20plugin_test_macos/20903
https://ci.chromium.org/ui/p/flutter/builders/prod/Mac%20plugin_test_macos/20902
https://ci.chromium.org/ui/p/flutter/builders/prod/Mac%20plugin_test_macos/20901

Recent test runs:
https://flutter-dashboard.appspot.com/#/build?taskFilter=Mac%20plugin_test_macos

Please follow https://github.com/flutter/flutter/blob/master/docs/infra/Reducing-Test-Flakiness.md#fixing-flaky-tests to fix the flakiness and enable the test back after validating the fix (internal dashboard to validate: go/flutter_test_flakiness).