L2capTest: Add common event handler and migrate test case

Migrated L2cap tests to use facade only:
test_disconnect_on_timeout
test_respond_to_echo_request
test_reject_unknown_command

Test: cert/run_cert_facade_only.sh
Bug: 14987419
Change-Id: I809a95fbf148f3784377d05e630e69539f8f93ef