* Re-enabled mock tests for H/3.

* Used IsMockQuicSupported to run the mock tests only on supported platforms.

* Disabling mock test completely.

* Hanging test fix experiment.

* Disabled some mock tests failing in CI.

* Fix #53632

* Annotate non-nullable _originAuthority

* Test inheriting original host for HTTP/3 Alt-Svc

* Alt-Svc tests enabled and fixed for H/3

* Addressed feedback.

Cause a connection abort when HTTP/2-only frame and settings types are received in HTTP/3. (Progress toward HTTP/3 draft 31 compat) (#43287)

Send h3-29 through h3-31 for ALPN as we are compatible with all of them.
Move some of the loopback test code into the stream class rather than connection class.
Add a basic test for SETTINGS frame receipt.
Fix some tests that were explicitly using Http3LoopbackServer and did not see the new "Mock" QUIC setting introduced in https://github.com/dotnet/runtime/pull/43076

* define HttpVersion.Version30 and use where appropriate

* add xmldoc to HttpVersion

* fix debug references

* readd HttpVersion30 to HttpClientHandlerTestBase

* revert changes to HttpClientHandlerTest.cs
Co-authored-by: NGeoffrey Kizer <geoffrek@windows.microsoft.com>

Update/fix MsQuic P/invoke layer.
Fix a race condition in MsQuicListener setting MsQuicConnection.Connected.
Some MsQuic cleanup.
Remove (un-reviewed) HttpVersion.Version30 and SslApplicationProtocol.Http3 APIs.
Fix Alt-Svc support.
Make Alt-Svc tests timeout properly.

Update generic tests to use a Version rather than boolean IsHttp11/IsHttp20, and update some HTTP/2 to work for HTTP/3.
Enable tests for HTTP/3, behind a conditional feature test.
Fix QPackDecoder lzcnt assuming an 8-bit test.
Rename test QPACK classes from QPackEncoder/QPackDecoder -> QPackTestEncoder/QPackTestDecoder to avoid naming confusion with product code classes.
Fix QPackTestDecoder bit flag checks.
Fix a double call to QuicConnection.CloseAsync(). Update to shutdown QuicConnection in a background task.
Fix test cert usage.