RFR: 8327460: Compile tests with the same visibility rules as product code

[Magnus Ihse Bursie]

Currently, our symbol visibility handling for tests are sloppy; we only handle it properly on Windows. We need to bring it up to the same levels as product code. This is a prerequisite for [JDK-8327045](https://bugs.openjdk.org/browse/JDK-8327045), which in turn is a building block for Hermetic Java.

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Commit messages:
 - 8327460: Compile tests with the same visibility rules as product code

Changes: https://git.openjdk.org/jdk/pull/18135/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18135&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8327460
  Stats: 303 lines in 38 files changed: 69 ins; 159 del; 75 mod
  Patch: https://git.openjdk.org/jdk/pull/18135.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18135/head:pull/18135

PR: https://git.openjdk.org/jdk/pull/18135