RFC: Enable Lit testing on Windows buildbot

Hello,

I am going to add this step in the nearest time.

Thanks

Galina

Galina,

I saw working on, thank you!

http://google1.osuosl.org:8011/builders/llvm-gcc-native-mingw32-win7/builds/1586/steps/run.build.step.test_llvm/logs/stdio

...Takumi