Implement GET channels endpoints for APIv4 (#5363)

* implement get channels endpoints and updated drivers and unittests

* removed channel deletion on tear down, removed manage permission on get channels endpoints, and updated utils to add constant channel length

* added constants for user, team and channel length, updated context to use the model functions

* make sure team name length should be less than the minimum length and revert underscore to team name validity

* changed post test condition from notfound to unauthorized

api4/apitestlib.go

@@ -278,11 +278,11 @@ func GenerateTestUsername() string {
 }
 
 func GenerateTestTeamName() string {
-	return "faketeam" + model.NewId()
+	return "faketeam" + model.NewRandomString(6)
 }
 
 func GenerateTestChannelName() string {
-	return "fakechannel" + model.NewId()
+	return "fakechannel" + model.NewRandomString(10)
 }
 
 func VerifyUserEmail(userId string) {