tests: Allow parallel running of tests

To allow running multiple instances of tests in parallel only few
changes were needed:

• Do not killall boothd on start
• Do not call ensure_boothd_not_running and rely on cleanup mechanism of

tests itself (works quite well). Another advantage is not rely on
(deprecated) netstat and perl one-liner.

• Generate "unique" port for each of the instance (based on PID)

This patch is (mostly) to allow building rpm (check section) on both i686
and x86_64 when only one physical machine is used (example reproducer is
Fedora Koji) for both architectures.

I was also thinking about having unique port for each test case where
working_config is needed, but such solution was more collisions prone
(and more invasive) than one port per tests instance.

Signed-off-by: Jan Friesse <jfriesse@redhat.com>