RFR: JDK-8296968: Update langtools tests to use @enablePreview [v3]

[Joe Darcy]

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

Joe Darcy has updated the pull request incrementally with one additional commit since the last revision:

  Implement review feedback.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/11149/files
  - new: https://git.openjdk.org/jdk/pull/11149/files/b4748553..faac4542

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11149&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11149&range=01-02

  Stats: 5 lines in 5 files changed: 0 ins; 1 del; 4 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