[foreign-memaccess] RFR: Fix bad use of MemorySegment::spliterator in TestSharedAccess

[Maurizio Cimadamore]

By accident, the patch for  https://git.openjdk.java.net/panama-foreign/pull/115contains a mistake on TestSharedAccess,
where the `MemorySegment::spliterator` is still accessed in a non-static fashion, thus preventing compilation.

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Commit messages:
 - Fix bad use of MemorySegment::spliterator in TestSharedAccess

Changes: https://git.openjdk.java.net/panama-foreign/pull/125/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/125/webrev.00
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/125.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/125/head:pull/125

PR: https://git.openjdk.java.net/panama-foreign/pull/125