Fix k5test prompts for Python 3

With Python 3, sys.stdout.write() of a partial line followed by
sys.stdin.readline() does not display the partial line.  Add explicit
flushes to make prompts visible in k5test.py.

https://github.com/krb5/krb5/commit/297535b72177dcced036b78107e9d0e37781c7a3
Author: Greg Hudson <ghudson@mit.edu>
Commit: 297535b72177dcced036b78107e9d0e37781c7a3
Branch: master
 src/util/k5test.py |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)