[test] Make jdt.core tests configurable to run only on LTS versions

By now it's obvious that jdt.core runs far too many tests every time we build. Even though the jenkins run tests only at specific levels, the builds run all of them. Every new test added (even if it is designed to run only from 1.8) will be run 15 times.

I am wondering if we should, at some point, just run only the LTS levels by passing a flag (or even by default). This is me thinking aloud. So, there could be reasons why this may not work.

Start by inspecting the jdt.core test configuration and the Jenkins test levels described in the issue. Determine how tests are currently selected across Java levels, then define and validate a configurable way to run only LTS levels without changing the existing Jenkins-specific behavior.

Written by the indexing model from the issue text.