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

[Athijegannathan Sundararajan]

On Fri, 24 Sep 2021 15:53:51 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

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

Marked as reviewed by sundar (Committer).

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