Add a helper for testing PKINIT and responder cb

https://github.com/krb5/krb5/commit/e87b20116528dd68299c7cb4783ba68bfe12a5f0
Author: Nalin Dahyabhai <nalin@redhat.com>
Committer: Greg Hudson <ghudson@mit.edu>
Commit: e87b20116528dd68299c7cb4783ba68bfe12a5f0
Branch: master
 src/tests/Makefile.in |   13 +-
 src/tests/deps        |    5 +
 src/tests/responder.c |  390 +++++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 404 insertions(+), 4 deletions(-)