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, for example by running
```
./run -t libvirt --list-tests --tests virsh.domuuid --no error_test
```
, to list the tests you needed exactly.

Signed-off-by: Hao Liu <hliu@redhat.com>