Added jacoco for test coverage

• added jacoco dependency to build.gradle.kts
• new jacocoReport task in :backend
• extended top level test task with a dependency to the jacocoReport

Contributes to https://gitlab.opencode.de/opencode-analyzer/management/-/issues/61

To test this: rerun ./gradlew test. The build folder in app/backend should contain a reports/jacoco folder containing the test coverage. Further test summary and test coverage are now automatically added to the MR.