RFR: 8287138: Make VerifyOption an enum class

[Thomas Schatzl]

Hi all,

  please review this change that make the `VerifyOption` enum an enum class as suggested in the review for [JDK-8189669](https://bugs.openjdk.java.net/browse/JDK-8189669).

Testing: local compilation, gha

Thanks,
  Thomas

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Commit messages:
 - 8287138: Make VerifyOption an enum class

Changes: https://git.openjdk.java.net/jdk/pull/8832/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8832&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8287138
  Stats: 28 lines in 8 files changed: 3 ins; 0 del; 25 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8832.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8832/head:pull/8832

PR: https://git.openjdk.java.net/jdk/pull/8832