Bluetooth: host: add testing API (disconnect one EATT channel)

To test fallback to remaining bearers PTS might request IUT to
disconnect one of the connected EATT channels, while the others remain
intact. Test function must be added, because we cannot create L2CAP
server on EATT PSM and manage this server as normal and have EATT
enabled at same time.

This is affecting GATT/SR/GAW/BV-14-C

Signed-off-by: Krzysztof Kopyściński <krzysztof.kopyscinski@codecoup.pl>