bta: le_audio_client_test: Fix ConfigureStream mock method

Missing CIS Connection Handle assignment in UnicastTest's
LeAudioGroupStateMachine::ConfigureStream to mock the
LeAudioGroupStateMachineImpl::ConfigureStream method.
This fix is needed by succeeding patches that add new
unit tests.

Bug: 357472821
Flag: Exempt; trivial change covered with unit tests
Test: atest bluetooth_le_audio_client_test
Change-Id: I12139d851f374bb13f1afe44cc412cbcb80566d9