Run engine unit tests on Windows presubmit checks

Although we build all unit tests on Windows, we only run very few of them (and we ran none of them before https://github.com/flutter/engine/pull/12423).

We need some very brave person to [enable all unit tests on Windows](https://github.com/flutter/engine/blob/master/.cirrus.yml#L118), and lead us to fix all the Windows unit tests failures.