tests/avocado: Cancel BootLinux tests in case there is no free port

tests/avocado: Cancel BootLinux tests in case there is no free port

The BootLinux tests are currently failing with an ugly python
stack trace on my RHEL8 system since they cannot get a free port
(likely due to the firewall settings on my system). Let's properly
check the return value of find_free_port() instead and cancel the
test gracefully if it cannot get a free port.

Message-Id: <20220228114325.818294-1-thuth@redhat.com>
Reviewed-by: Beraldo Leal <bleal@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>