refactor: Add `mem` module to allow tests to override allocators. #3548

Job	Run time
docker-bootstrap-node	28s
docker-toxcore-js	3m 23s
docker-clusterfuzz	1m 32s
docker-esp32	55s
docker-fuzzer	15s
docker-win64	13s
docker-win32	13s
docker-bootstrap-node-websocket	0s
	6m 59s

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