Skip tests requiring fork() when no fork() is available

If you build perl on Windows without -DPERL_IMPLICIT_SYS (which I do, in
order to enable -DPEL_MALLOC, which seems faster than using the system
malloc()) then you don't get the fork() emulation and several of Plack's
tests fail.

This commit skips those tests in the same manner as various other CPAN
modules do in this case. This allows a normal "cpan install ..." of
Plack or anything depending on it (e.g. Dancer) to succeed without having
to "force" anything.