RFR: 8283774: TestZoneOffset::test_immutable should ignore ZoneOffset::rules

[Claes Redestad]

- Add capability to ignore fields explicitly when checking for immutability of classes in java.time
- Use this to avoid a test failure due the new `rules` cache in `ZoneOffset`
- Remove `TestZoneOffset` from problem list.

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Commit messages:
 - TestZoneOffset::test_immutable should ignore ZoneOffset::rules

Changes: https://git.openjdk.java.net/jdk/pull/7989/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7989&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8283774
  Stats: 12 lines in 3 files changed: 6 ins; 3 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7989.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7989/head:pull/7989

PR: https://git.openjdk.java.net/jdk/pull/7989