libcasper/cap_grp tests: Reset the group database handle

Some tests verify that the capgrp capability does not permit calls to
setgrent(3), but all tests need to ensure that they reset the
capability's group database handle, otherwise the local process and
casper process will be out of sync.

The cap_pwd tests already handle this.

Sponsored by: The FreeBSD Foundation

(cherry picked from commit 1a05d9519de62cc0f7719a995fded0926d8b2747)