RFR: Restrict generational mode to adaptive heuristic

[William Kemper]

On Fri, 24 Mar 2023 18:59:50 GMT, William Kemper <wkemper at openjdk.org> wrote:

> This is a small change to Shenandoah, but it prevents us from running the tests with the generational mode enabled via a jtreg vm option. This requires us to properly integrate the generational mode with the jtreg tests. There are many test files changed, but the `generational` mode is now handled in the same fashion as other modes (`iu`, `passive`). There are two additional advantages to these changes:
> * We no longer need to run the test suite twice (once in the default mode and again with `-XX:ShenandoahGCMode=generational`).
> * Expanded test coverage, as some tests would spawn their own VMs with permutations of mode and heuristic options that ignored the generational mode.

Converted to draft while I investigate a few failures.

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PR Comment: https://git.openjdk.org/shenandoah/pull/233#issuecomment-1485521072