[MM-37585] (#28634)

* add tests

* pass userID to the function instead of the user object.

* remove concurrent login simulation

---------

Co-authored-by: Mattermost Build <build@mattermost.com>

server/channels/app/channels.go

@@ -76,6 +76,8 @@ type Channels struct {
 
 	postReminderMut  sync.Mutex
 	postReminderTask *model.ScheduledTask
+
+	loginAttemptsMut sync.Mutex
 }
 
 func NewChannels(s *Server) (*Channels, error) {