Add tests for per-request preauth data scoping

Add a test harness which interleaves calls for multiple initial creds
contexts using the same library context.  Add a test case to
t_preauth.py using the new harness and the test preauth module to
verify that modreq pointers are correctly tracked.

https://github.com/krb5/krb5/commit/c0b25fe282355d4f329418956b9c6295780af633
Author: Greg Hudson <ghudson@mit.edu>
Commit: c0b25fe282355d4f329418956b9c6295780af633
Branch: master
 .gitignore               |    1 +
 src/tests/Makefile.in    |   23 +++++---
 src/tests/icinterleave.c |  124 ++++++++++++++++++++++++++++++++++++++++++++++
 src/tests/t_preauth.py   |   13 +++++
 4 files changed, 152 insertions(+), 9 deletions(-)