test/udev-test.pl: use computed devnode name

test/udev-test.pl: use computed devnode name

More often than not, the created devnode is the basename of the
sysfs entry. The "devnode" device may be used to override the
auto-detected node name.

Permissions and major/minor number are now verified on the devnode
itself, not on symlinks.

For those tests where exp_name is set to the computed devnode name,
the explicit "exp_name" can be removed. "exp_name" is only required for
symlinks.

This allows separate testing for devnodes and symlinks an a follow-up
patch.