Fix intermittents tests for macos desktop

`.github/workflows/test.yml` contains:
```yaml
# TODO: Enable macos desktop when supported
#- run: flutter test integration_test/webcrypto_test.dart -d macos
# working-directory: ./example
```

We really should enable integration tests on macos desktop. If anyone is interested in figuring how to get this working on Github Actions that would be great.

Contributions are highly appreciated, I tried in https://github.com/google/webcrypto.dart/pull/74, but there is probably a tiny thing not working.