lit.py: Give up and disable the new shared-output.py test on Windows

"No such file or directory: C:\\...\\tests\\Output\\shared-output.py.tmp/Output/Shared/SHARED.tmp"

And yet other forward-slashes don't seem to be causing the same
problem. I'll see if I can get ahold of a Windows machine to poke at
this directly later.

llvm-svn: 315792