refactor: Add `mem` module to allow tests to override allocators. #3578

Job	Run time
docker-bootstrap-node	1m 32s
docker-fuzzer	9s
docker-clusterfuzz	42s
docker-toxcore-js	2m 29s
docker-esp32	3m 13s
docker-win32	8s
docker-win64	3m 13s
docker-bootstrap-node-websocket	15s
	11m 41s

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