Fixing flaky test TestUnlinkGroupTeam (#14019)

There is a race at the DB level where the goroutine c.App.SyncRolesAndMembership
would set the the SchemeAdmin attribute to false. If this happens after
UpdateUserToTeamAdmin happens and before the Login, then the user does not
have the required permissions to unlink the group and it fails.

Thanks to @streamer45 for spotting the issue.

Co-authored-by: mattermod <mattermod@users.noreply.github.com>

api4/group_test.go

@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"net/http"
 	"testing"
+	"time"
 
 	"github.com/mattermost/mattermost-server/v5/model"
 	"github.com/stretchr/testify/assert"
@@ -236,6 +237,7 @@ func TestUnlinkGroupTeam(t *testing.T) {
 
 	response = th.Client.UnlinkGroupSyncable(g.Id, th.BasicTeam.Id, model.GroupSyncableTypeTeam)
 	assert.NotNil(t, response.Error)
+	time.Sleep(2 * time.Second) // A hack to let "go c.App.SyncRolesAndMembership" finish before moving on.
 	th.UpdateUserToTeamAdmin(th.BasicUser, th.BasicTeam)
 	ok, response := th.Client.Logout()
 	assert.True(t, ok)