Kudos to you for taking this stance about 'design test' from the hiring side. As a person on the other side I go a step further and refuse to do even paid design tests. From the FAQ section of my portfolio website:

Will you do a paid test job?

I completely understand you’d be comfortable if you could see my design for a small portion of your app/website before engaging with me for the full project. Also, by this way, both of us can get a feel for our way of working and communication. However, I find it difficult to justify the effort I put into a test job. Designing a small section of your product may look like a small task but when we take into account the time required for research, planning, brainstorming, sketching out ideas, analyzing them, establishing a visual style, etc., it becomes a biggish project (in case you think I don’t have to do all these steps and just ‘design’ that section then it becomes beautifying, not designing).

A test project of size, say, 1/25th of the full project usually requires a lot more than 1/25th of the effort required to complete the full project. So, my fee would be most certainly higher than what you think it’d be for the 1/25th part. So, after I size up the work and send you an estimate most likely it’ll be turned down :-) Even if you’re ready to pay what I ask, there is always the possibility of not getting to work on the full project, in which case I’d hate to see my effort go waste. For this reason I never take up test projects.

Alternatively, you can go through the case studies of my previous projects to get an idea of how I work and read my previous clients’ testimonials to get an idea of how it was to work with me; you can also talk to some of my previous clients if you feel so. I’ve never had an unhappy client and most of the work I get is from repeat clients.