RFR: 8303582: Reduce duplication in test code (mainline)

[Jorn Vernee]

Port of: https://github.com/openjdk/panama-foreign/pull/804 which reduces duplication in the test code by switching test value generation over to a common method in `NativeTestHelper`.

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Commit messages:
 - remove byteWidthOfPrimitive for now
 - 8302990: Reduce duplication in test code

Changes: https://git.openjdk.org/jdk/pull/12856/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12856&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8303582
  Stats: 503 lines in 13 files changed: 138 ins; 272 del; 93 mod
  Patch: https://git.openjdk.org/jdk/pull/12856.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12856/head:pull/12856

PR: https://git.openjdk.org/jdk/pull/12856