RFR: Restrict generational mode to adaptive heuristic [v4]

[William Kemper]

> 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 one additional commit since the last revision:

  Restore reference CAS barrier test for generational mode

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/233/files
  - new: https://git.openjdk.org/shenandoah/pull/233/files/12e402ab..0a55ba70

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=233&range=03
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=233&range=02-03

  Stats: 26 lines in 1 file changed: 26 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/shenandoah/pull/233.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/233/head:pull/233

PR: https://git.openjdk.org/shenandoah/pull/233