Spek should be compatible with JUnit Platform suites

[marcphilipp]

JUnit 5.8 introduced @Suite classes and a new engine to execute them. Spek is currently incompatible with them because it does not use the UniqueId passed in to TestEngine.discover for the children of the engine descriptor and instead constructs its own:
https://github.com/spekframework/spek/blob/2.x/spek-runner-junit5/src/main/kotlin/org/spekframework/spek2/junit/SpekTestDescriptor.kt#L20

[rpalcolea]

Hi @raniejade

Having proper UniqueId support would help people using Gradle to use features such as test retries, predictive test selection and test distribution

Wondering if you have any thoughts on this?