[foreign-memaccess+abi] RFR: Improve test coverage in the jdk.internal.layout package [v4]

[Per Minborg]

> This PR adds some tests and minor code refactoring.

Per Minborg has updated the pull request incrementally with one additional commit since the last revision:

  Remove spurious copyringt year update

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Changes:
  - all: https://git.openjdk.org/panama-foreign/pull/790/files
  - new: https://git.openjdk.org/panama-foreign/pull/790/files/d77641f1..358397dd

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=panama-foreign&pr=790&range=03
 - incr: https://webrevs.openjdk.org/?repo=panama-foreign&pr=790&range=02-03

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/790.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/790/head:pull/790

PR: https://git.openjdk.org/panama-foreign/pull/790