Add test cases for preauth fallback behavior

Add test cases for preauth fallback behavior

Add options to icred for performing optimistic preauth and setting
preauth options, and for choosing between the normal and stepwise
interfaces.  Add options to the test preauth module to allow induced
failures at several points in processing, factoring out some padata
manipulation functions into a new file to avoid repeating too much
code.  Add test cases to t_preauth.py using the new facilities to
exercise and verify several preauth fallback scenarios.  Amend the
tryagain test case in t_pkinit.py to look for more trace log messages.

ticket: 8537