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
Test-SharedFork'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
Test-SharedFork or anything depending on it (e.g. Plack) to succeed
without having to "force" anything.