It seems there is no automated tests on Windows (I am not sure this is being done manually before each release).
Assuming from this comment, https://github.com/apache/spark/pull/14743#issuecomment-241473794, It seems we have Windows infrastructure in the AMPLab Jenkins cluster.
It seems pretty much important because as far as I know we should manually test and verify some patches related with Windows-specific problem.
I was thinking a combination with Travis CI and Docker with Windows image. Although this might not be merged, I will try to give a shot with this (at least for SparkR) anyway (just to verify some PRs I just linked above).
I would appreciate it if I can hear any thoughts about this.