Spring Enable memory leak detection for tests with Netty 5 Buffer

Fix several memory leaks in tests.

Comment From: violetagg

@rstoyanchev @poutsma PTAL

Comment From: violetagg

@rstoyanchev PTAL I moved the ownership change from Netty5WebSocketSessionSupport#toMessage to WebSocketMessage#retain

Comment From: violetagg

@rstoyanchev Thanks