refactor: Add `mem` module to allow tests to override allocators. #3501

Job	Run time
docker-clusterfuzz	1m 31s
docker-bootstrap-node	27s
docker-esp32	43s
docker-toxcore-js	2m 27s
docker-fuzzer	13s
docker-win32	18s
docker-win64	8s
docker-bootstrap-node-websocket	0s
	5m 47s

Provide feedback