le_audio: Add missing sync on main loop while closing GATT

After timing out LE Audio group transition a GATT connection is closed,
to avoid reces with cleaning connection ID, a sync should be performed
before test cleanup.

Bug: 278026474
Tag: #feature
Test: atest UnicastTest#SpeakerStreamingTimeout
Change-Id: I87da21371390e075805a94ba1ce58fee146c171b