RFR: 8335932: GenShen: Fix old heuristic unit test

[William Kemper]

This isn't really a unit test and it must be run with Shenandoah enabled, but it exercises a hard to reach code path. The test has suffered some bit-rot* of late that needs to be addressed.

* We no longer set soft-max-capacity on the old generation.

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Commit messages:
 - Fix old heuristic test

Changes: https://git.openjdk.org/shenandoah/pull/458/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=458&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8335932
  Stats: 11 lines in 1 file changed: 5 ins; 3 del; 3 mod
  Patch: https://git.openjdk.org/shenandoah/pull/458.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/458/head:pull/458

PR: https://git.openjdk.org/shenandoah/pull/458