tests/installed: Make reboot task less racy

tests/installed: Make reboot task less racy

This took a whole lot of experimentation.  I hit upon the idea
of doing a `systemctl stop sshd` to avoid the situation where we
might ssh back into the system while it's in the process of shutting
down.

Ultimately the other fix is disabling `ControlMaster`; see
for example: https://github.com/ansible/ansible/issues/17935

Closes: #1548
Approved by: cgwalters