tests: Ensure 'unshare' works before relying on it.

Fixes <https://bugs.gnu.org/35642>.
Reported by Josh Holland <josh@inv.alid.pw>.

* tests/guix-pack-relocatable.sh: Before invoking 'unshare' at the
bottom, add "if unshare -r true" condition.
* tests/guix-pack.sh: Likewise.