If you've created a libgdx project via the setup tool, then you should have a gradle build that has everything you need in terms of libgdx. You can then copy over SimpleTest1 to your project. And finally, you need to add the spine-libgdx runtime.
You can do that by either copying the contents spine-runtimes/spine-libgdx/spine-libgdx/src to your project's core/src folder (so you end up with core/src/com/esotericsoftware/spine/), or you can add the dependency in your build.gradle file as you said you did.
If that still fails, then what I said in my first reply still holds: you are trying to load a .skel file exported from a Spine Editor version that is != the Spine Runtimes version you are using. That does not work. The versions have to match. If you use com.esotericsoftware.spine:spine-libgdx:4.0.18.1, then your skeleton must have been exported with Spine Editor version 4.0.x.
