RFR: 8349504: Support platform-specific JUnit tests in jpackage

[Alexey Semenyuk]

Move junit tests to `share` subfolder.

Adjust `TEST.properties` to grant full access to classes in all `jdk.jpackage` module packages. Otherwise, jtreg picks access config from the parent [`TEST.properties`](https://github.com/openjdk/jdk/blob/aad6664bb6d2b311b3e0cb056afaa9b6534bdbbb/test/jdk/tools/jpackage/TEST.properties), which allows full access to `jdk.jpackage.internal` package only.

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

Commit messages:
 - Move shared junit tests from "junit" to "junit/share" folder in preparation for adding platform-specific junit tests

Changes: https://git.openjdk.org/jdk/pull/23479/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23479&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8349504
  Stats: 7 lines in 10 files changed: 5 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/23479.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23479/head:pull/23479

PR: https://git.openjdk.org/jdk/pull/23479