RFR: 7946: core/common.test is not running any tests

[Christoph Langer]

Currently, probably since the latest refactoring, the common.test project does not execute any of its tests.

Let's fix it (in pom.xml) along with some other cleanups to a few test cases.

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Commit messages:
 - JMC-7946

Changes: https://git.openjdk.org/jmc/pull/448/files
 Webrev: https://webrevs.openjdk.org/?repo=jmc&pr=448&range=00
  Issue: https://bugs.openjdk.org/browse/JMC-7946
  Stats: 78 lines in 8 files changed: 22 ins; 18 del; 38 mod
  Patch: https://git.openjdk.org/jmc/pull/448.diff
  Fetch: git fetch https://git.openjdk.org/jmc pull/448/head:pull/448

PR: https://git.openjdk.org/jmc/pull/448