The test used to hang on waitForRead(), sometimes, which underneath
involve a poll()+read() syscall pair.
When this happened, the IMAP data came together with the proxy data on a
previous poll()+read() call and the proxy code had already consumed it.
We now wait for data only if data is not already available.
Fixes: QTBUG-96345
Pick-to: 6.2
Change-Id: I084f5d1268a5091ea614fcec91c8d356dcb90d9f
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>