XFAIL pexpect tests on Windows.

At some point we will need to either provide a pexpect equivalent
on Windows, or provide some other method of doing out-of-process
tests.

Even with a pexpect replacement, it may be worth re-evaluating
some of these tests to see if they would be better served as
in-process tests.  The larger issue of coming up with a pexpect
replacement on Windows is tracked in http://llvm.org/pr22274.

llvm-svn: 226614