Avoid keyctl purge in keyring ccache tests

keyctl purge was added in keyutils 1.5 (released in March 2011).  Use
keyctl unlink to clean up keys instead, as it is more universal.

(cherry picked from commit 94da4584645475272abec6259d1666e34bd59594)

https://github.com/krb5/krb5/commit/22cfc33cc61415adccf1d55618d8058d607e6930
Author: Greg Hudson <ghudson@mit.edu>
Committer: Tom Yu <tlyu@mit.edu>
Commit: 22cfc33cc61415adccf1d55618d8058d607e6930
Branch: krb5-1.12
 src/lib/krb5/ccache/t_cccol.py |   30 ++++++++++++++++--------------
 src/tests/t_ccache.py          |   20 ++++++++++++--------
 2 files changed, 28 insertions(+), 22 deletions(-)