Add standalone HTTP stress test (dotnet/corefx#39021)

Add standalone HTTP stress test (dotnet/corefx#39021)

* Add standalone HTTP stress test

This is a simple standalone stress test that launches Kestrel in-proc, creates an HttpClient, and then concurrently makes lots of requests of varying kinds from the client to the server.  It's already discovered a plethora of issues in our HTTP/2 implementation, so I'm checking it in hopes that it can help to enable others to discover more.

* Address PR feedback

Commit migrated from https://github.com/dotnet/corefx/commit/3298cd55f840a8a3fab4450ac64ff3547073f651