Why is it offset from center? To stress test the steering?

The design of the full rocket doesn't have a single center engine. Instead, it has three engines at the center.

This test article uses the same layout for engines, but with one engine instead of three. So that one engine is offset from the center.

Their flight software is already capable of handling the multiple engine out-scenario and compensating for it, so there's no real reason for them to center the engine for the test article.

The 'thrust puck' the engine is mounted to is designed for multiple engines, none of which are in the centre.

They are only using one engine this test, but are testing the flight-design thrust puck (as opposed to some interim structure with a centre mount for the single engine).

It's designed for 3 sea level engines and 3 vacuum engines to be installed in a radially symmetric fashion... But they only installed 1 sea level engine on this prototype.

There's some center-of-gravity and shifting of things as you burn up fuel. Not an expert, so here's where the streamer talks a bit about it, including why the space shuttle did it too: https://youtu.be/NJR4gZBLMNw?t=1195