RFR: 7904078: Mute JUnit logging to warning level

[Christian Stein]

Please review this small change to configure JUnit's built-in logging severity to warning level.

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Commit messages:
 - 7904078: Mute JUnit logging to warning level

Changes: https://git.openjdk.org/jtreg/pull/289/files
  Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=289&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7904078
  Stats: 7 lines in 3 files changed: 6 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jtreg/pull/289.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/289/head:pull/289

PR: https://git.openjdk.org/jtreg/pull/289