[foreign-memaccess] Integrated: Unsafe ParallelSum test is broken

[Maurizio Cimadamore]

The numbers for the unsafe parallel sum tests are artificially low; after some debugging I realized that the test was
not summing up all components. I then plugged in the unsafe fork join action into our existing memory segment
spliterator test, and verified that indeed that fork join action was violating all assertions.

I then fixed all the issues, verified that assertions were satisfied and then plugged the fixed action back into the
microbenchmark.

Unsurprisingly, the perf numbers of the unsafe part are now in sync with those obtained from the segment counterpart.

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Commit messages:
 - Fix ParallelSum/unsafe test

Changes: https://git.openjdk.java.net/panama-foreign/pull/288/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/288/webrev.00
  Stats: 9 lines in 1 file changed: 4 ins; 0 del; 5 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/288.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/288/head:pull/288

PR: https://git.openjdk.java.net/panama-foreign/pull/288