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.

google-auth-library-php/.github/workflows/tests.yml

Line 34 in bff9f2d

run: vendor/bin/phpunit ${{ matrix.os == 'windows-latest' && '--filter GCECredentialsTest' || '' }}