refactor: Add `mem` module to allow tests to override allocators. #3568

Job	Run time
docker-bootstrap-node	1m 34s
docker-clusterfuzz	42s
docker-esp32	2m 32s
docker-fuzzer	22s
docker-toxcore-js	2m 59s
docker-win32	8s
docker-win64	9s
docker-bootstrap-node-websocket	17s
	8m 43s

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