[jdk16] RFR: 8259634: MemorySegment::asByteBuffer does not respect spatial bounds

[Maurizio Cimadamore]

The byte buffers created from heap segments do not honor the javadoc - which says that the resulting buffer size should be equal to MemorySegment::byteSize, and that the buffer position should be zero.

The issue is that the NIO routine we have added to create heap buffers is using the wrong constructor, which doesn't do what we need. The fix is to simply use the proper, more complete constructor.

I've also re-enabled an unrelated test which was missing the @Test annotation.

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Commit messages:
 - Fix loic to turn segments into buffers

Changes: https://git.openjdk.java.net/jdk16/pull/109/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk16&pr=109&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8259634
  Stats: 30 lines in 2 files changed: 28 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk16/pull/109.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk16 pull/109/head:pull/109

PR: https://git.openjdk.java.net/jdk16/pull/109