tests: userspace: work around x86_64 bug

Exceptions on x86_64 are incorrectly implemented, and if
a preemptible thread faults, and in its overridden
k_sys_fatal_error_handler() does something which invokes
a scheduling point (such as here where we give semaphores),
the thread will be swapped out on the per-CPU exception stack
and probably explode when it is switched back in.

For now, change the faulting thread priority to co-op so this
doesn't happen.

Workaround for #21462

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>