RFR: 8337847: [GenShen] jdk.test.whitebox.Whitebox.isObjectInOldGen misreports

[Y. Srinivas Ramakrishna]

**Summary**

The whitebox test method `jdk.test.whitebox.Whitebox.isObjectInOldGen` was not correctly implemented for GenShen. It reports only whether the object is in the heap, not whether it's in the old gen, which is its specification.

Upon fixing the implementation for GenShen, a regression test was found to be too tight in insisting that a full GC should promote objects into the old generation. The test was fixed, for the case of GenShen, such that it would not insist on this behavior.

**Testing:**

- [x] GHA
- [x] hotspot/jtreg

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Commit messages:
 - Relax a test that insists that objects are promoted after a full gc,
 - Fix WB_isObjectInOldGen for GenShen.

Changes: https://git.openjdk.org/shenandoah/pull/466/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=466&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8337847
  Stats: 11 lines in 2 files changed: 9 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/shenandoah/pull/466.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/466/head:pull/466

PR: https://git.openjdk.org/shenandoah/pull/466