build-sys: test/sys depends on test/sys.tar.xz

build-sys: test/sys depends on test/sys.tar.xz

Fixes:
$ ls -ld test/sys
drwxr-xr-x. 12 vagrant vagrant 4096 Jan  6  2012 test/sys

$ git pull
Updating d4337c1..710a885
Fast-forward
 ...
 test/sys.tar.xz                             | Bin 261380 -> 165116 bytes
 test/udev-test.pl                           |  67 ++++++++++++-----

$ sudo make check TESTS=test/udev-test.pl
...
FAIL: test/udev-test.pl

$ rm -r test/sys
$ make test/sys
$ sudo make check TESTS=test/udev-test.pl
...
PASS: test/udev-test.pl