[foreign-memaccess+abi] RFR: Improve test coverage in the jdk.internal.layout package [v2]

[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:

  Improve tests

-------------

Changes:
  - all: https://git.openjdk.org/panama-foreign/pull/790/files
  - new: https://git.openjdk.org/panama-foreign/pull/790/files/ad328f0b..45dbb843

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=panama-foreign&pr=790&range=01
 - incr: https://webrevs.openjdk.org/?repo=panama-foreign&pr=790&range=00-01

  Stats: 59 lines in 1 file changed: 25 ins; 28 del; 6 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