RFR: JDK-8309142: Refactor test/langtools/tools/javac/versions/Versions.java

[Joe Darcy]

Test refactoring to reduce future maintenance: when source version gets a new language feature, a single enum constant with an example gets added to test all source versions against that sample.

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Commit messages:
 - JDK-8309142: Refactor test/langtools/tools/javac/versions/Versions.java

Changes: https://git.openjdk.org/jdk/pull/14387/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14387&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8309142
  Stats: 340 lines in 1 file changed: 89 ins; 181 del; 70 mod
  Patch: https://git.openjdk.org/jdk/pull/14387.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14387/head:pull/14387

PR: https://git.openjdk.org/jdk/pull/14387