RFR: 7856: Fix errors in Eclipse projects after JMC-7769

[Christoph Langer]

[JMC-7769](https://bugs.openjdk.org/browse/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.

The fix is to set up org.openjdk.jmc.common.test like a general library project, only using src/main/java -> target/classes.

Because it is a module of missioncontrol.core.tests, the property `fail.if.no.tests=false` needs to be set, otherwise the build fails because tests would be expected.

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Commit messages:
 - JMC-7856

Changes: https://git.openjdk.org/jmc/pull/413/files
 Webrev: https://webrevs.openjdk.org/?repo=jmc&pr=413&range=00
  Issue: https://bugs.openjdk.org/browse/JMC-7856
  Stats: 15 lines in 3 files changed: 8 ins; 5 del; 2 mod
  Patch: https://git.openjdk.org/jmc/pull/413.diff
  Fetch: git fetch https://git.openjdk.org/jmc pull/413/head:pull/413

PR: https://git.openjdk.org/jmc/pull/413