Cert: Defer unknown DCID in L2capTests

* Sometimes, L2CAP Configuration Request may arrive before
  scid_to_channel map is updated, making it not possible to
  handle such request
* Deferring the request until the connection is made fixes
  the problem
* Adding threading.Lock does not help due to the global interpreter
  lock

Bug: 307877650
Test: m bluetooth_stack_with_facade && gd/cert/run --clean L2capTest
Change-Id: I038d33c7332bbde241bfa68e61ec6f977836e73a