Add test program for gss_process_context_token

Add a new test program t_pcontok to exercise
gss_process_context_token, and run it from t_gssapi.py.

(cherry picked from commit bfb472ff67c00da2f2b0d0ada1af57a2c4493a11)

https://github.com/krb5/krb5/commit/97f96c5f74b069d7bc66bc2c5fe35c904b5e7a03
Author: Greg Hudson <ghudson@mit.edu>
Committer: Tom Yu <tlyu@mit.edu>
Commit: 97f96c5f74b069d7bc66bc2c5fe35c904b5e7a03
Branch: krb5-1.13
 .gitignore                   |    1 +
 src/tests/gssapi/Makefile.in |   20 +++--
 src/tests/gssapi/t_gssapi.py |    1 +
 src/tests/gssapi/t_pcontok.c |  202 ++++++++++++++++++++++++++++++++++++++++++
 4 files changed, 215 insertions(+), 9 deletions(-)