Integrated: JDK-8307954: Update string template regression tests to be robust on release updates

[Joe Darcy]

Change the string template diagnostic tests to use the preferred ${jdk.version} idiom rather than hard-coding a particular release value.

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Commit messages:
 - JDK-8307954: Update string template regression tests to be robust on release updates

Changes: https://git.openjdk.org/jdk/pull/13936/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13936&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8307954
  Stats: 6 lines in 6 files changed: 0 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/13936.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13936/head:pull/13936

PR: https://git.openjdk.org/jdk/pull/13936