Add k5test expected_msg, expected_trace

Add k5test expected_msg, expected_trace

In k5test.py, add the optional keyword argument "expected_msg" to
methods that run commands, to make it easier to look for substrings in
the command output.  Add the optional keyword "expected_trace" to run
the command with KRB5_TRACE enabled and look for an ordered series of
substrings in the trace output.