aco/tests: Use _exit in child process

aco/tests: Use _exit in child process

Since the child process doesn't call exec(), exit() attempted to run
atexit handlers registered by the parent process. This could result in
the child process hanging in exit() if there were still disk cache
threads alive when the parent process called fork(). (The CI runners
hit this multiple times when running tests in strace)

Fixes: 6a246f5c6d51 "aco/tests: Fix deadlock for too large test lists"
Reviewed-by: Tony Wasserka <tony.wasserka@gmx.de>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9472>