RFR: 8271073: Improve testing with VM option VerifyArchivedFields

[Ioi Lam]

- Changed the definition of `VerifyArchivedFields` from a whacky use of `bool` to an `int` and properly define its three levels:
  - 0: No verification 
  - 1: Basic verification with VM_Verify (no side effects)
  - 2: Detailed verification by forcing a GC (with side effects)
- Changed the default value to 0. The functionality checked by this flag has been very stable so there's no need to verify it in every single test case.
- Enabled `-XX:VerifyArchivedFields=1` for all CDS test cases.
- Added a new test case for `-XX:VerifyArchivedFields=2` .
- Also added comments about that this flag is suppose to check for.

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Commit messages:
 - 8271073: Improve testing with VM option VerifyArchivedFields

Changes: https://git.openjdk.java.net/jdk/pull/5514/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5514&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8271073
  Stats: 85 lines in 5 files changed: 72 ins; 0 del; 13 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5514.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5514/head:pull/5514

PR: https://git.openjdk.java.net/jdk/pull/5514