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

[Per Minborg]

This PR adds some tests and minor code refactoring.

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Commit messages:
 - Unbump copyright years
 - Remove debug code
 - Add tests
 - Update copyright years
 - Merge foreign-memaccess+abi
 - Add tests and rewrite equals() methods

Changes: https://git.openjdk.org/panama-foreign/pull/790/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=790&range=00
  Stats: 108 lines in 6 files changed: 73 ins; 18 del; 17 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