What is stopping us from launching seed payloads to these planets with some way to continually water them? Wouldn't that be a way to theoretically test the survivability of the planet without sending people?

It's possible there are some forms of Earth life that might, just maybe, survive on the Martian surface in the summer season. But probably not. There are a few issues with the idea.

First, whether lichen can survive in full sun with a watering robot to tend it, doesn't really tell us anything about whether humans can survive on Mars. (Lichen is also growing inside the Chernobyl reactor.)

Second, hopefully we haven't contaminated Mars with Earth life yet. If we have, it would make it much harder to find Martian life buried somewhere, or trace evidence of previous life on Mars in the deep past. It's official policy, and has been since the start of the space era, of all the space agencies (which they follow with varying degrees of actual commitment in practice, I assume) to avoid contamination of any planet with Earth life. Until we can conclusively rule out life or past history of life on a particular world, we should probably keep to this policy.

We already know that plants will not survive in martian soil regardless of watering them. For the amount of effort/energy/consumables needed to turn martian soil into something that can grow plants, it would be easier to just send some soil. Or, skip all that and go totally hydroponic. Hydroponics makes economic sense here on earth already. On mars, hydroponics is exponentially easier than traditional farming methods.

The first habitats will probably not use any martian resources. First uses will probably be as construction materials. Then possibly as a fuel source. Those are the high-mass uses. Things like growing food/oxygen don't require much mass and so are far down the list.