[lworld] RFR: 8294312: [lworld] Add Objects.isIdentityObject(obj)

[Roger Riggs]

Add `java.util.Objects.isIdentityObject(obj)` and update tests.

Correct j.l.Class.isIdentity() and .isValue() to correctly identify all arrays as identity objects.
(The modifiers for array classes do have not reliable ACC_IDENTITY or ACC_VALUE bits).

Updated ValhallaFeaturesTest to use junit.

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Commit messages:
 - 8294312: [lworld] Add Objects.isIdentityObject(obj)

Changes: https://git.openjdk.org/valhalla/pull/770/files
 Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=770&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294312
  Stats: 76 lines in 5 files changed: 53 ins; 6 del; 17 mod
  Patch: https://git.openjdk.org/valhalla/pull/770.diff
  Fetch: git fetch https://git.openjdk.org/valhalla pull/770/head:pull/770

PR: https://git.openjdk.org/valhalla/pull/770