test: backup /etc/udev/udev.conf only if it exists

test: backup /etc/udev/udev.conf only if it exists

On Fedora systemd recently moved all of its configuration files to
/usr/lib/ [0], so make sure we account for this case as well.

[   42.450325] testsuite-17.sh[800]: + mkdir -p /run/udev/rules.d
[   42.466504] testsuite-17.sh[800]: + cp -f /etc/udev/udev.conf /etc/udev/udev.conf.bckp
[   42.503348] testsuite-17.sh[802]: cp: cannot stat '/etc/udev/udev.conf': No such file or directory

[0] https://src.fedoraproject.org/rpms/systemd/c/29eb35530b29232eed65718d0cd96d67cd7ffd6b?branch=rawhide