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.

(back ported from commit bfb472ff67c00da2f2b0d0ada1af57a2c4493a11)

https://github.com/krb5/krb5/commit/d8cb443634fd83d11680822b023aec45c3adac26
Author: Tom Yu <tlyu@mit.edu>
Commit: d8cb443634fd83d11680822b023aec45c3adac26
Branch: krb5-1.12
 .gitignore                   |    1 +
 src/tests/gssapi/Makefile.in |   18 ++--
 src/tests/gssapi/t_gssapi.py |    1 +
 src/tests/gssapi/t_pcontok.c |  202 ++++++++++++++++++++++++++++++++++++++++++
 4 files changed, 214 insertions(+), 8 deletions(-)