Restyle refactor: Add `mem` module to allow tests to override allocators. #3522

Job	Run time
docker-bootstrap-node	30s
docker-clusterfuzz	1m 35s
docker-fuzzer	8s
docker-esp32	2m 37s
docker-toxcore-js	3m 1s
docker-win64	12s
docker-win32	13s
docker-bootstrap-node-websocket	0s
	8m 16s

Provide feedback