[MM-26837] attempt at fixing flaky test on mysql-5.6 (#15110)

Summary

- Attempt at fixing the AutocompleteInTeam/empty_string test which is flaky on MySQL-5.6. This is likely an index_merge_intersection optimization issue. See https://community.mattermost.com/core/pl/uzgygwa9e3d4pns14ayfs7mobw
    for more discussion. The fix here simply modifies the where clause to change the query plan so index_merge_intersection is not used, while also not introducing a temporary table or a filesort.

Ticket Link

- https://mattermost.atlassian.net/browse/MM-26837

store/sqlstore/channel_store.go

@@ -2584,7 +2584,7 @@ func (s SqlChannelStore) GetMembersForUserWithPagination(teamId, userId string,
 }
 
 func (s SqlChannelStore) AutocompleteInTeam(teamId string, term string, includeDeleted bool) (*model.ChannelList, *model.AppError) {
-	deleteFilter := "AND c.DeleteAt = 0"
+	deleteFilter := "AND Channels.DeleteAt = 0"
 	if includeDeleted {
 		deleteFilter = ""
 	}
@@ -2597,7 +2597,7 @@ func (s SqlChannelStore) AutocompleteInTeam(teamId string, term string, includeD
 		JOIN
 			PublicChannels c ON (c.Id = Channels.Id)
 		WHERE
-			c.TeamId = :TeamId
+			Channels.TeamId = :TeamId
 			` + deleteFilter + `
 			%v
 		LIMIT ` + strconv.Itoa(model.CHANNEL_SEARCH_DEFAULT_LIMIT)