[foreign-memaccess+abi] RFR: ValueLayouts: Improve test coverage and more

[Per Minborg]

This PR:

* Adds tests to `ValueLayout` methods
* Simplifies an `equals()` method
* Changes from exception throwing to assert for parameters that cannot be changed client-side

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Commit messages:
 - Change to asserts, rewrite equals and add tests

Changes: https://git.openjdk.org/panama-foreign/pull/786/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=786&range=00
  Stats: 64 lines in 2 files changed: 28 ins; 21 del; 15 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/786.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/786/head:pull/786

PR: https://git.openjdk.org/panama-foreign/pull/786