[foreign-memaccess+abi] RFR: Split tests

[Per Minborg]

This PR suggests splitting up a test class into two and only run the slow one on 64-bit systems.

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Commit messages:
 - Split tests

Changes: https://git.openjdk.org/panama-foreign/pull/908/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=908&range=00
  Stats: 153 lines in 2 files changed: 102 ins; 51 del; 0 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/908.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign.git pull/908/head:pull/908

PR: https://git.openjdk.org/panama-foreign/pull/908