Conditionally run CNG tests only on supported platforms.

@victorhurdugaci - this replaces https://github.com/aspnet/DataProtection/pull/42.  The primary difference is that this is conditioned on "does the OS support the p/invoke feature I'm about to use?" rather than trying to whitelist / blacklist OSes.
This repo is not using the latest xunit. The conditional only works with that. See this change: https://github.com/aspnet/KRuntime/commit/bf65c35e35d3d95a86bb164513ba66be5fe11387