Consider running CI under Windows

[jbduncan]

As is currently done in google-java-format, I think it's worth testing Guava against Windows using AppVeyor, or at least I think it's worth for the Guava team to consider looking into it.

Currently, Guava makes no guarantees that it will run successfully on Windows, as explained in the README:

IMPORTANT WARNINGS
...
5, We unit-test and benchmark the libraries using only OpenJDK 1.7 on Linux. Some features, especially in com.google.common.io, may not work correctly in other environments.

I do understand that there would need to be a number of changes or improvements before Guava can build successfully on Windows (#2130), so I suspect this would be a long-term thing to tackle.

[cpovirk]

@cgdecker pointed out that such testing would have caught the problem he identified with the Guava 32.0.0 reimplementation Files.createTempDir and FileBackedOutputStream. In fairness, he does also remind me of the same IMPORTANT WARNING that you quoted above :)

[jbduncan]

@cpovirk Oh no! Yeah, the potential incompatibility between the new implementations and Windows occurred to me earlier today. 😆