Consider moving Kestrel shared source test attributes to src\Shared

[wtgodbe]

e.g. https://github.com/dotnet/aspnetcore/blob/wtgodbe/3IIS/src/Servers/Kestrel/shared/test/TransportTestHelpers/HttpSysHttp3SupportedAttribute.cs and https://github.com/dotnet/aspnetcore/blob/wtgodbe/3IIS/src/Servers/Kestrel/shared/test/TransportTestHelpers/MsQuicSupportedAttribute.cs, which are used in tests across Kestrel, IIS, and HTTPSys. See #35934 (comment)

We've moved this issue to the Backlog milestone. This means that it is not going to be worked on for the coming release. We will reassess the backlog following the current release and consider this item at that time. To learn more about our issue management process and to have better expectation regarding different types of issues you can read our Triage Process.