Fix input race condition in t_skew.py

Fix input race condition in t_skew.py

In two of the kinit tests run by t_skew.py, we expect kinit to exit
before reading the password.  If we supply a password input for those
commands, we can fail with a broken pipe exception if the master
process tries to write the password after the slave process exits.

Also correctly check the output of the last kinit invocation.

(cherry picked from commit 2457bf66c466321dd36cd3c76bc36bb589d31587)

ticket: 8147 (new)
version_fixed: 1.11.6
status: resolved