extra-tests: add test-guests to prevent test error

extra-tests: add test-guests to prevent test error

extra-tests is depending on tests-guests.
We need guests/*.img when doing extra-tests.

/work/git/libguestfs/run valgrind --log-file=/work/git/libguestfs/tests/extra/valgrind.log --leak-check=full --error-exitcode=119 --suppressions=/work/git/libguestfs/tests/extra/suppressions ../../cat/virt-cat -a ../guests/fedora.img /etc/fstab >/dev/null
../guests/fedora.img: No such file or directory
make[1]: *** [test-tools-internal] Error 1
make[1]: Leaving directory `/work/git/libguestfs/tests/extra'

Signed-off-by: Wanlong Gao <gaowanlong@cn.fujitsu.com>