`utbot-junit-contest` requires separate gradle build

[Domonion]

Description

Currently UtBot Gradle build relies on JVM 11 for some projects.
In utbot-junit-contest we need JDK 8 to compile tests, which leads to divergation as there are no sun.misc packages in JDK 11. To support that we require building tests with JDK 8, but overall Gradle pipeline working on JDK 11.

This require either:

1. Completely different gradle project - the easiest, but the worst solutions as it would complicate developing and debugging a lot.
2. Rewrite gradle script - may be it is possible to use special worker for project or specify JDK version for compilation.

Anyway, for scripts to run correctly it is requires JDK 8 available on machine - the solution will probably involve docker containers rebuild.

Requires: #1741