refactor: Add `mem` module to allow tests to override allocators. #3554

Job	Run time
docker-bootstrap-node	1m 25s
docker-esp32	50s
docker-clusterfuzz	1m 28s
docker-fuzzer	8s
docker-toxcore-js	2m 39s
docker-win64	14s
docker-win32	13s
docker-bootstrap-node-websocket	12s
	7m 9s

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