Create multi-message WebSockets test

[imiric]

While working on fixing the flaky TestSentReceivedMetrics (PR #1149) we noticed that all our current WS tests only test sending one message and disconnecting.

This issue is to add one or more tests where more than one message is passed, which will require a change in websocketEchoHandler() to avoid disconnecting after the first message.

Since the WS handler can impact test flakiness, make sure to test this change thoroughly.

[cooliscool]

Hi 👋 I'm interested to take this up.