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.

https://github.com/krb5/krb5/commit/94da4584645475272abec6259d1666e34bd59594
Author: Greg Hudson <ghudson@mit.edu>
Commit: 94da4584645475272abec6259d1666e34bd59594
Branch: master
 src/lib/krb5/ccache/t_cccol.py |   30 ++++++++++++++++--------------
 src/tests/t_ccache.py          |   20 ++++++++++++--------
 2 files changed, 28 insertions(+), 22 deletions(-)