test: install /etc/default/knot if available as well

The knot.service on Ubuntu Jammy loads an env file which we didn't
install, causing the service to fail:

```
knot.service: Will spawn child (service_enter_start_pre): /usr/sbin/knotc
knot.service: Failed to load environment files: No such file or directory
knot.service: Failed to run 'start-pre' task: No such file or directory
knot.service: Failed with result 'resources'.
knot.service: Service will not restart (restart setting)
```

test/TEST-75-RESOLVED/test.sh

@@ -27,6 +27,7 @@ test_append_files() {
     image_install /lib/tmpfiles.d/knot.conf
     image_install "${ROOTLIBDIR:?}/system/knot.service"
     image_install -o /etc/dbus-1/system.d/cz.nic.knotd.conf
+    image_install -o /etc/default/knot
 
     # Copy over our configuration
     mkdir -p "${workspace:?}/var/lib/knot/zones/" "${workspace:?}/etc/knot/"