rewrite :readme-examples from spek to junit5

[robstoll]

Platform (all, jvm, js): all
Extension (none, kotlin 1.3): none

Code related feature

the readme-example project executes tests and write the output of the failure into the README.md of Atrium. It is based on Spek and since Spek doesn't advance, has bugs which makes it impossible for Atrium to upgrade respectively, we should move away from spek entirely. This includes also the readme-example project. Instead of providing an own TestEngine (which uses SpekTestEngine) we should write just normal tests and use a TestExecutionListener or similar (something which is able to turn a failing test into a successful one and vice versa) to achieve the same.

This isn't a good first issue but might be fun to tackle. In the end, executing ./gradlew :readme-example:build should generate the same output in README.md (see for instance <ex-first> in README.md and the corresponding FirstExampleSpec

Once we don't rely on spek, we can also update junit #1677