Replace `exit` by `_exit` in child.#70

See https://stackoverflow.com/questions/5422831/what-is-the-difference-between-using-exit-exit-in-a-conventional-linux-fo and section 25.4 in `The Linux Programming Interface` by Michael Kerrisk. The `exit` library call was causing our tests to become extremely slow when running with ASAN on arm64 and ppc64le architectures. Replacing with `_exit` fixed the issue. Haven't exactly pinpointed the issue, but the problem is related to the child and the parent both flushing the parent's stdio buffers and calling the exit handlers. Signed-off-by: Mathieu Borderé <[email protected]>