Add test coverage for GSS context export/import

Pass the -export flag to gss-server in t_gss_sample.py, in order to
test context export and import for each of the mechanisms.

(cherry picked from commit bee2d867248b24c627da4c2ef270c8de15fd96f9)

https://github.com/krb5/krb5/commit/e802afa475291762b57ba2d27ce5c81dbaec2c07
Author: Greg Hudson <ghudson@mit.edu>
Committer: Tom Yu <tlyu@mit.edu>
Commit: e802afa475291762b57ba2d27ce5c81dbaec2c07
Branch: krb5-1.14
 src/appl/gss-sample/t_gss_sample.py |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)