Don't remember seeing the single rider line at Test Track and very rarely see one at Everest. But after my experience with the obnoxious kid (and he really spoiled the ride for me), I'm going to ask the Sorting CM to please let me sit by myself.
We go as a group of 3. It is hit or miss that the single rider in our group (usually me) is grouped with a stranger. Never for Thunder Mountain (we always ask to ride in the back and the last row is narrower than the others in the cars). However, there was a confused couple once who thought they were going to squeeze in with me - I gave them a look of panic (one of them was not, well, slim) and inquired why they were going to sit with me. The CM stopped them and told them "the next car!"