Fix errors in Eclipse projects after JMC-7769

JMC-7769 cleaned up the project layout for missioncontrol core, especially the test projects. However, after that patch, an Eclipse IDE setup shows errors in test projects of the application, e.g. in org.openjdk.jmc.rjmx.test.

This is caused by an issue in project setup of core/tests/org.openjdk.jmc.common.test, which is a prerequisite to some other test projects. Class files are generated into the target/test-classes folder but Eclipse only exposes target/classes to dependent projects.