[FAB-18333] Fix panic in cluster/comm#TestRenewCertificates
[FAB-18333] Fix panic in cluster/comm#TestRenewCertificates

In the unit test TestRenewCertificates, node 2 is restarted and its certificates
are rotated.

It is tested that node 1 fails to re-connect to the old certificate until it is reconfigued.
However, since it is not aware of the new reconfiguration, the old gRPC stream might be stale
and it might not be detected, and the expected error that is checked results in a nil pointer panic.

This PR aims to fix the nil pointer panic, by repeatedly trying to use the stream until an error is returned.
This ensures that the gRPC stream is un-usable and when the server starts with a new certificate
it will still be un-usable.

Change-Id: I038624907ebab198bb3a193b137a47b2e7c970d0
Signed-off-by: Yacov Manevich <632344b5da9bd743456189af4db4b56472defb8c@il.ibm.com>

manish-sethi/fabric-sidedb,manish-sethi/fabric-sidedb,hyperledger/fabric,stemlending/fabric,manish-sethi/fabric,manish-sethi/fabric-sidedb,manish-sethi/fabric-sidedb,manish-sethi/fabric,stemlending/fabric,manish-sethi/fabric-sidedb,manish-sethi/fabric-sidedb,stemlending/fabric,hyperledger/fabric,manish-sethi/fabric-sidedb,jimthematrix/fabric,stemlending/fabric,jimthematrix/fabric