Skip parallel tests on Windows

We don't know _why_ the tests fail but we know they do. See
http://www.cpantesters.org/cpan/report/da23a490-9853-1014-a85a-71fc2a825c07
for a good example of how this fails.

I suspect that the answer may be patches for one of Test::Builder,
Parallel::ForkManager, or TCM itslef.