[lworld] RFR: 8331602: [lworld] Test using -XX:+EnableNullableFieldFlattening should not be run in product mode

[Frederic Parain]

NullableFlatFieldTest and UnsafeTest needs EnableNullableFieldFlattening, which is a develop flag, not available on product build. EnableNullableFieldFlattening is a temporary flag that will be removed once all the VM supports nullable flat fields. For now, those two tests will only be run with debug builds.

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Commit messages:
 - Restrict tests to debug mode

Changes: https://git.openjdk.org/valhalla/pull/1095/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1095&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8331602
  Stats: 2 lines in 2 files changed: 2 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/valhalla/pull/1095.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1095/head:pull/1095

PR: https://git.openjdk.org/valhalla/pull/1095