[foreign-jextract] RFR: Fix jextract test after arena allocator API change

[Maurizio Cimadamore]

As the subject says, this simple change tweaks one jextract test to use the new arena allocator method name.

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Commit messages:
 - Fix test to use new arena allocator method name

Changes: https://git.openjdk.java.net/panama-foreign/pull/584/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=584&range=00
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/584.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/584/head:pull/584

PR: https://git.openjdk.java.net/panama-foreign/pull/584