refactor: Add `mem` module to allow tests to override allocators. #3507

Job	Run time
docker-bootstrap-node	24s
docker-clusterfuzz	1m 28s
docker-esp32	43s
docker-fuzzer	8s
docker-toxcore-js	2m 28s
docker-win32	7s
docker-win64	11s
docker-bootstrap-node-websocket	0s
	5m 29s

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