[lworld] RFR: 8304168: Modify CDS tests to disable -XX:-EnableValhalla with --enable-preview

[Roger Riggs]

When --enable-preview is true, the patching of some java.base classes as value classes disables CDS. 
Subsequently the CDS tests fail.

For a CDS and a JDI test, disable Valhalla when --enable-preview is set so java.base is not patched with value classes.

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Commit messages:
 - Modify CDS tests to disable -XX:-EnableValhalla with --enable-preview

Changes: https://git.openjdk.org/valhalla/pull/832/files
 Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=832&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8304168
  Stats: 4 lines in 2 files changed: 2 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/valhalla/pull/832.diff
  Fetch: git fetch https://git.openjdk.org/valhalla pull/832/head:pull/832

PR: https://git.openjdk.org/valhalla/pull/832