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.

ticket: 8034 (new)
target_version: 1.13.1
tags: pullup