tests: posix: fs: increase ztest stack size from 1k to 2k

The default stack size was causing qemu_arc/qemu_arc_em and
qemu_arc/qemu_arc_hs to fail when a dirent object was
stack-allocated.

Increase the default stack size to accomodate for additional
usage.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>