Test owl_zephyr_smartstripped_user

Signed-off-by: Anders Kaseorg <andersk@mit.edu>
owl_zephyr_smartstripped_user: Simplify, fixing overlapping strcat

Previously, the strcat(out, realm+1) call might read from the same
place it’s writing to, which is invalid and could break if strcat is
optimized in certain ways.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>