Cluster test: catch FLUSHALL errors on node reset.

FLUSHALL will fail on read-only slaves, but there the command is not
needed in order to reset the instance with CLUSTER RESET so errors can
be ignored.

tests/cluster/tests/includes/init-tests.tcl

@@ -21,7 +21,7 @@ test "Cluster nodes are reachable" {
 
 test "Cluster nodes hard reset" {
     foreach_redis_id id {
-        R $id flushall
+        catch {R $id flushall} ; # May fail for readonly slaves.
         R $id cluster reset hard
         R $id config set cluster-node-timeout 3000
     }