Add tests for key rotation and 32-bit keytab kvnos

In t_keytab.py, test that kvnos no longer wrap after 255 or 32767, that
they do wrap from 65535 to 1, and that kadmin ktrem preserves the more
recent key after a wraparound.

Also test edge cases of the 32-bit keytab kvno extension using
hand-crafted keytab entries.

https://github.com/krb5/krb5/commit/3c873481596653f5a35be1fffc268847867e89e7
Author: Greg Hudson <ghudson@mit.edu>
Commit: 3c873481596653f5a35be1fffc268847867e89e7
Branch: master
 src/tests/t_keytab.py |   81 ++++++++++++++++++++++++++++++++++++++++++-------
 1 files changed, 70 insertions(+), 11 deletions(-)