Add an assertion to verify the size of the ACL queue.

Summary: acl_queue is never checked for emptiness after waiting for callback which can lead to undefined behaviour when calling pop() and front(). Add asserts to validate queue before continuing the test.

Test: run the test and callback timeout is accurately caught by asserts.

Change-Id: I8a922b2d33c0b8d31b100d2ef532584f4d491922
Signed-off-by: Abhishek Gadewar <abhishekgadewar@meta.com>