Fix running proxypac test on qemu

Do not override '--no-sandbox' on boot2qt. Starting resolver service
on qemu is bonkers and ends up in unsupported calls during threading
checks. Adding '--no-zygote' solves the issue, however introduces
race condition and deadlocks on resolver service start, which would
make test flaky. The deadlock issue is not reproducible without qemu.
Therefore to workaround the issue run test in single process mode.

Task-number: QTBUG-63346
Change-Id: I33a1079776edecddf3d7653db6c2ecc3a868b1a1
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>