Prepare for launch

@exercism/kotlin-test-runner Here is basic implementation of auto-test runner. There are some things I'm gonna improve, but you are welcomed to check the code and share your thoughts.

I'll add high-level docs later, there are some kdocs and I tried to make code quite simple.

PR: #4 if you prefer to check changes as a Pull Request.

P.S. Let's use JUnit5 in new exercises! :)

TODO:
- [x] Dockerfile
- [ ] **(!)** Add docs
- [ ] Decrease docker image size
- [ ] **(!)** Add more tests
- [ ] **(!)** Find a way to capture user's in
- [ ] Use Gradle Tooling API
- [ ] Use `Path` instead of `File` to run tests on in-memory FS (e.g. [jimfs](https://github.com/google/jimfs))
- [ ] Try graal / jlink / kotlin native to improve performance and size