Add tests with LoopbackServer for web sockets

[greenEkatherine]

We should increase test scenarios for web sockets that are running with LoopbackServer -- first of all, for web sockets over HTTP/2 and then for HTTP/1.1.

Now many tests are running only in outerloop, which runs on request and not in every CI. This option is unavailable for web sockets over HTTP/2 because the external server has not yet supported it.

To achieve more test coverage, Http2LoopbackServer should be adapted to work with web sockets i.e., it should have a special stream wrapper to decode received data easily.

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Issue Details

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We should increase test scenarios for web sockets that are running with LoopbackServer -- first of all, for web sockets over HTTP/2 and then for HTTP/1.1.

Now many tests are running only in outerloop, which runs on request and not in every CI. This option is unavailable for web sockets over HTTP/2 because the external server has not yet supported it.

To achieve more test coverage, Http2LoopbackServer should be adapted to work with web sockets i.e., it should have a special stream wrapper to decode received data easily.

Author:	greenEkatherine
Assignees:	greenEkatherine
Labels:	area-System.Net.Http, test-enhancement
Milestone:	-