RFR: 7508: Use preferred way for test-jars

[Jean-Philippe Bempel]

based on https://maven.apache.org/plugins/maven-jar-plugin/examples/create-test-jar.html
I have modified the way test jars are structured:
we put all classes in src/main/java instead of src/test/java
and changed scope from test to compile accordingly
we now don't need to create explicitly test-jar neither reference them
as is.
Help to build/run/debug correctly in Intellij

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Commit messages:
 - add resources source folder for common.test
 - update .classpath
 - update build.properties
 - use preferred way for test-jars

Changes: https://git.openjdk.java.net/jmc/pull/354/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=354&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-7508
  Stats: 119 lines in 166 files changed: 1 ins; 78 del; 40 mod
  Patch: https://git.openjdk.java.net/jmc/pull/354.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/354/head:pull/354

PR: https://git.openjdk.java.net/jmc/pull/354