Storage testbench should "close connection" after every request

ISSUE FACED:
Retry conformance tests were failing for dotnet on being deployed on Github CI. The issue was flaky in the test suite. The error was:
" System.Net.Http.HttpRequestException : An error occurred while sending the request.
---- System.IO.IOException : The response ended prematurely. "

POSSIBLE SOLUTION:
Change the testbench to use a different worker type that supports keep-alive connections (and probably extend the default keep-alive time to 15 seconds or similar)
OR
Work out why HttpClient isn't reliably detecting the connection closing, and fix that (so that it just automatically uses a new connection)

Please refer to b/275276459 for more details and examples