[bt][gap] Clear callbacks before tearing down TestController tests

Delete callbacks that may contain dangling references to test data when
tearing down tests that use TestController.

Also move transaction count in BrEdrConnectionManagerTest from test
cases to the harness.

BT-671
Test: fx set x64 --variant="asan/bt-host-unittests"
fx run-test bluetooth_tests -t bt-host-unittests
(with local changes for BT-672 BT-673 TC-333)

Change-Id: I33d080a431345864bc5778b1a9755cc7edde3147
4 files changed