Add more PlatformColor tests for PlatformColor and PlatformNSColor when converting to/from WebCore::Color, and when using IPC serialization. This also adds more NSShadow tests. Most of these were written based on investigations during this change: Generate serialization of NSShadow https://bugs.webkit.org/show_bug.cgi?id=268963 <rdar://107553244> https://commits.webkit.org/274870@main
<rdar://problem/123166372>
Pull request: https://github.com/WebKit/WebKit/pull/24693