RFR: 8303705: Field sleeper.started should be volatile JdbLockTestTarg.java

[Leonid Mesnik]

Field has been made volatile.

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Commit messages:
 - Variable is made volatile.

Changes: https://git.openjdk.org/jdk/pull/13010/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13010&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8303705
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/13010.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/13010/head:pull/13010

PR: https://git.openjdk.org/jdk/pull/13010