test/recipes/80-test_pkcs12.t: handle lack of Win32::API.

So far check for availability of Win32::API served as implicit check
for $^O being MSWin32. Reportedly it's not safe assumption, and check
for MSWin32 has to be explicit.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5416)