feat(tests): enable Windows testing for the whole test suite

[bshaffer]

In our Github Actions workflow, we add a filter for GCECredentialsTests when running on Windows. This is because the tests don't run as expected on Windows. I doubt this is because of a bug in the library - it's probably just a bug / platform difference in the test! But we should look into it, and enable Windows testing across the whole test suite.

https://github.com/googleapis/google-auth-library-php/blob/bff9f2d01677e71a98394b5ac981b99523df5178/.github/workflows/tests.yml#L34