RFR: 8303489: Add a test to verify classes in vmStruct have unuque vtables

[Alex Menkov]

Unique vtables for classes in vmStruct data is a requirement for SA to correctly detect hotspot classes.
The fix adds test to verify this requirement.

The test fails as expected on Windows if VM is built without RTTI (see JDK-8302817)

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Commit messages:
 - UniqueVtableTest

Changes: https://git.openjdk.org/jdk/pull/12820/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12820&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8303489
  Stats: 177 lines in 1 file changed: 177 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/12820.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12820/head:pull/12820

PR: https://git.openjdk.org/jdk/pull/12820