How Do I Change The Test Explorer Window To Display Test Source Information Below, Rather Than Beside?

What I have found is that Visual Studio automatically decides where to place the divide for this window. You can test this out by undocking the Test Explorer window and then dragging the corner around to resize it. I'm not certain what the ratio is but the details area will snap around either at the bottom or to the right side somewhere around the window being about 1.5 times taller than wider.

This was tested with the community edition of VS 2013. Maybe the "higher" editions allow you to configure this but it doesn't look like the lower editions let you. Hope that helps!

Using 2019 Community Edition, I was able to undock and achieve this.

Then I put the undocked item back into my main IDE window and it remained like that.