[foreign-memaccess+abi] RFR: TestScopedOperations does not work as expected

[Maurizio Cimadamore]

When Julia started to work on the `MemorySegment::overlap` API, she noticed (thanks!) an issue with the TestScopedOperation. Essentially this test checks that calling methods on various objects should be scope-safe - that is, it shouldn't be possible to call certain methods (e.g. MemorySegment::address) if the scope associated with the object is closed, or from a thread other than the owning thread.

The test had a fatal flaw, which was probably introduced when we moved to the `ResourceScope` API. Since the scoped operation creates the desired object (e.g. a segment) and then performs an operation on it, and since that operation is performed _after_ the scope has been closed, as a result, all tests trivially fail as expected, since the object cannot be created in the first place (the scope is closed!!).

The solution is to separate the logic for creating the object from the logic for testing the scoped operation. In fixing the test I realized that the test was running two combinations which didn't make sense (prefixAllocator::allocate, and segment::spliterator, neither of which is scoped), and found a bug in the VaList implementation - for VaList::skip, which was missing a scope check (which this patch also addresses).

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Commit messages:
 - Fix TestScopedOperations

Changes: https://git.openjdk.java.net/panama-foreign/pull/587/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=587&range=00
  Stats: 50 lines in 6 files changed: 20 ins; 7 del; 23 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/587.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/587/head:pull/587

PR: https://git.openjdk.java.net/panama-foreign/pull/587