RFR: CODETOOLS-7903516: work around `this-escape` warnings

[Jonathan Gibbons]

Please review changes to make some classes final, to avoid effectively false-positive `this-escape` warnings.

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Commit messages:
 - CODETOOLS-7903516: work around `this-escape` warnings

Changes: https://git.openjdk.org/jtreg/pull/165/files
 Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=165&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903516
  Stats: 10 lines in 8 files changed: 0 ins; 0 del; 10 mod
  Patch: https://git.openjdk.org/jtreg/pull/165.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/165/head:pull/165

PR: https://git.openjdk.org/jtreg/pull/165