Fixed errcheck issues in server/channels/api4/post_test.go (#29201)

server/.golangci.yml

@@ -70,7 +70,6 @@ issues:
         channels/api4/config_local.go|\
         channels/api4/config_test.go|\
         channels/api4/data_retention.go|\
-        channels/api4/post_test.go|\
         channels/api4/preference_test.go|\
         channels/api4/reaction_test.go|\
         channels/api4/role.go|\