RFR: 7951: Factor out commonly used test helper classes from org.openjdk.jmc.common.test into dedicated test library module

[Christoph Langer]

Triggered by an issue in project core/tests/org.openjdk.jmc.common.test, I came up with a little refactoring.

So, org.openjdk.jmc.common.test is somewhat of a hybrid project. It contains both, some common test scaffolding classes plus some actual tests for the org.openjdk.jmc.common module.

According to our special setup, which is not really like what maven would expect, we have all sources, also for the tests in the src/main/java folder. With a little tweak,
that is specifying <testSourceDirectory>${project.basedir}/src/main/java</testSourceDirectory> in the pom.xml, we can direct maven to consume the tests from there instead
of the usually expected src/test/java subfolder. However, when updating the eclipse project configuration from that pom.xml, the project would not export the test helper
classes to other modules within eclipse any more.

I'm hereby factoring out the test helper classes into a new module called core/tests/org.openjdk.jmc.testlib. This module will then only serve as a test helper library but
not contain any actual tests.

Furthermore I include some other cleanups in project and manifest files, e.g. removal of .settings files which are .gitignored because they would be generated when importing
the JMC projects into Eclipse.

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Commit messages:
 - JMC-7951

Changes: https://git.openjdk.org/jmc/pull/451/files
 Webrev: https://webrevs.openjdk.org/?repo=jmc&pr=451&range=00
  Issue: https://bugs.openjdk.org/browse/JMC-7951
  Stats: 920 lines in 102 files changed: 195 ins; 368 del; 357 mod
  Patch: https://git.openjdk.org/jmc/pull/451.diff
  Fetch: git fetch https://git.openjdk.org/jmc pull/451/head:pull/451

PR: https://git.openjdk.org/jmc/pull/451