Fix flaky cluster pubsubshard test in 26-pubsubshard (#14962)

In the "PUBSUB channels/shardchannels" test, we call sunsubscribe
without channels, but the number of loops in
consume_subscribe_messages() is determined by the size of channels.
When channels are empty, the loop will loop 0 times and will not read
the sunsubscribe response message returned by the server.
This means that when verifying the channel length, the previous command
might not have been complete yet, so this PR added a read after
sunsubscribe.

tests/cluster/tests/26-pubsubshard.tcl

@@ -123,6 +123,7 @@ test "PUBSUB channels/shardchannels" {
     assert_equal {3} [llength [$publishclient pubsub shardchannels]]
 
     sunsubscribe $subscribeclient
+    $subscribeclient read
     set channel_list [$publishclient pubsub shardchannels]
     assert_equal {2} [llength $channel_list]
     assert {[lsearch -exact $channel_list "\{channel.0\}2"] >= 0}