standalone_test: Enable only filter when listing tests

Currently when running with option `--list-tests`, we can use `--no`
to remove some tests, but we can't apply an `only` filter.

With this modification, we can apply an `only` filter by running
```
./run -t libvirt --list-tests --tests virsh.domuuid
```
to list only the tests you needed.

Signed-off-by: Hao Liu <hliu@redhat.com>
Merge pull request #1777 from Hao-Liu/fix_list_tests

standalone_test: Enable only filter when listing tests