L2cap scheduler: Implement high priority channel

Add a high priority queue for these channels in scheduler.

Introduce MockQueue to help test with BidiQueueEnd callbacks.

Test: cert/run
Test: bluetooth_test_gd
Tag: #gd-refactor
Bug: 141555841
Change-Id: I3b60002f0d2ac2fb4d3307717dce128ccb06247c