RFR: Restrict generational mode to adaptive heuristic [v3]

[Aleksey Shipilev]

On Wed, 29 Mar 2023 15:43:25 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.
>
> William Kemper has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - Use named local variables and add comments to improve readability
>  - Revert changes to compiler regression tests

Everything is good now, except for `TestReferenceCAS`.

test/hotspot/jtreg/gc/shenandoah/compiler/TestReferenceCAS.java line 56:

> 54:  */
> 55: 
> 56: import java.lang.reflect.Field;

This one is actually not a regression test, it is similar to `TestClone` that verifies the generic support for Shenandoah barriers. we should keep it here.

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Marked as reviewed by shade (Committer).

PR Review: https://git.openjdk.org/shenandoah/pull/233#pullrequestreview-1363434982
PR Review Comment: https://git.openjdk.org/shenandoah/pull/233#discussion_r1152148102