RFR: JDK-8296968: Update langtools tests to use @enablePreview

[Joe Darcy]

Update langtools tests to use JTreg's @enablePreview tag, where appropriate.

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Commit messages:
 - JDK-8296968: Update langtools tests to use @enablePreview

Changes: https://git.openjdk.org/jdk/pull/11149/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11149&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8296968
  Stats: 133 lines in 32 files changed: 15 ins; 6 del; 112 mod
  Patch: https://git.openjdk.org/jdk/pull/11149.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11149/head:pull/11149

PR: https://git.openjdk.org/jdk/pull/11149