test: Ensure client rpclogger is set on RPC only client. (#26443)

If a test encounters an RPC error using the test client, it will
panic as the rpc logger is not set when it attempts to log the
error.

client/testing.go

@@ -107,6 +107,7 @@ func TestRPCOnlyClient(t testing.TB, cb func(c *config.Config), srvAddr net.Addr
 	client := &Client{
 		config:           conf,
 		logger:           testLogger,
+		rpcLogger:        testLogger.Named("rpc"),
 		shutdownCh:       make(chan struct{}),
 		EnterpriseClient: newEnterpriseClient(testLogger),
 	}