RFR: CODETOOLS-7902925: jcstress: Fix new Sonar warnings

[Aleksey Shipilev]

After recent work, Sonar complains about AbstractThread.

-------------

Commit messages:
 - Fix AbstractThread

Changes: https://git.openjdk.java.net/jcstress/pull/52/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jcstress&pr=52&range=00
  Issue: https://bugs.openjdk.java.net/browse/CODETOOLS-7902925
  Stats: 4 lines in 1 file changed: 3 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jcstress/pull/52.diff
  Fetch: git fetch https://git.openjdk.java.net/jcstress pull/52/head:pull/52

PR: https://git.openjdk.java.net/jcstress/pull/52