Disable lldb-mi tests on Windows.

Most were already XFAIL'ed, but the reason for the XFAIL is that
we don't have a suitable pexpect implementation on Windows.  This
isn't going to change unintentionally, so there is no reason to
XFAIL them as opposed to just skip them.

llvm.org/pr22274 tracks finding a suitable pexpect module for
Windows, which should fix many of these issues.  llvm.org/pr24452
tracks the larger issue of making the entire lldb-mi test suite
work on Windows, of which finding a pexpect module is just one
component.

llvm-svn: 244951