[foreign-memaccess+abi] RFR: 8312186: TestStringEncodingFails for UTF-32

[Jorn Vernee]

Add support for `UTF_32`

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Commit messages:
 - elaborate comment in test
 - add support for UTF_32

Changes: https://git.openjdk.org/panama-foreign/pull/847/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=847&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8312186
  Stats: 39 lines in 2 files changed: 33 ins; 1 del; 5 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/847.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign.git pull/847/head:pull/847

PR: https://git.openjdk.org/panama-foreign/pull/847