Add test file for cyclic weakrefs, where most or all refs to a parent are weak.

This test would have failed on Win32 due to Perl_hv_backreferences_p()
not being a public api. On linux this would not matter, on Win32 it
does.

Related:

Sereal Commit d4e6ec026f5669f2cb9555753b2787541c30281f
    https://github.com/Sereal/Sereal/commit/d4e6ec026f5669f2cb9555753b2787541c30281f

Sereal Issue #73
    https://github.com/Sereal/Sereal/issues/73

Perl RT Ticket #123473
    https://rt.perl.org/Ticket/Display.html?id=123473

Perl commit eaab56493bda8d2c9e499f01433ea2da9b29f6e5
    http://perl5.git.perl.org/perl.git/commit/eaab56493bda8d2c9e499f01433ea2da9b29f6e5
require EU:MM 7.0

Apparently old EU::MMs do not build properly. Avoid the problem by requiring a newer
EU:MM.
Release v3.003_003 - Test for weakref, and build changes