Fix failing Release mode unit tests

[maksimkim]

HttpUtilityTests:

• Align list of reserved characters with https://datatracker.ietf.org/doc/html/rfc3986#section-2.2 in HttpUtility implementation
• Use capital-case characters for percentage-encoding
• Stop using UrlEncoder in HttpUtility.Encode method in Release mode

Http2ConnectionHandlerTest:

• mock FrameWriter.WriteGoAwayAsync with Task.Delay to force scheduling channel.Close

SingleThreadEventLoopTest:

• mark two tests validating behavior of ExecuteAfterEventLoopIteration as DEBUG-only since the method itself is DEBUG-only.

misc:

• Use proper array pool reference in ArrayPooledByteBuffer tests

[tomkerkhove]

/azp run

[tomkerkhove]

/azp run

[tomkerkhove]

Re-running to find flaky tests again, but this one failed in previous run: https://msazure.visualstudio.com/ApiManagement/_build/results?buildId=106249508&view=ms.vss-test-web.build-test-results-tab&runId=1852304777&resultId=106704&paneView=debug

[tomkerkhove]

/azp run