RFR: 8338695: GenShen: Clean up jtreg tests

[William Kemper]

* Reformat new jtreg tests for generational mode
* Run tests with verifier enabled first
* Include generational mode in barrier enable/disable tests (and fix passive mode)

## Testing

* GHA
* internal tests
* make exploded-test TEST="hotspot_gc_shenandoah"

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Commit messages:
 - Fix compilation errors, don't let passive mode run with card barrier enabled
 - Fix compilation errors
 - Clean up tests

Changes: https://git.openjdk.org/shenandoah/pull/480/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=480&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8338695
  Stats: 496 lines in 17 files changed: 123 ins; 196 del; 177 mod
  Patch: https://git.openjdk.org/shenandoah/pull/480.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/480/head:pull/480

PR: https://git.openjdk.org/shenandoah/pull/480