Fix flaky test register_enqueue_with_half_empty_queue

The TestQueue was deleted while still running a task.
This is because Handler.cc is not looking for running task when being
deleted

Fix: 264602833
Test: atest bluetooth_test_gd_unit64
Change-Id: I2131c14226cc2b086e8d1c6eeacaf1f9d6876da2