Add master key rollover tests in k5test framework

Add master key rollover tests in k5test framework

Add a new script t_mkey.py using the k5test framework.  Test the fixes
for #6507, #7685, and #7686 as well as basic functionality and
old-stashfile compatibility.

dump.16 was created by running "kdb5_util create -s -P footes" and
"kdb5_util dump dumpfile" with krb5 1.6.  The key from the resulting
stash file was extracted and placed in the struct.pack() call in the
new test script.