RFR: 8356040: java/util/PluggableLocale/LocaleNameProviderTest.java timed out [v2]

[Justin Lu]

> Please review this PR which reduces the amount of combinations tested by `LocaleNameProviderTest`.
> 
> This test was exhaustively testing over 1000x1000 `Locale` combinations which was expensive and caused a timeout in tier 5. Narrowing down the tested locales to Japanese language ones can reduce our tested input to 9x9 and provides most of the same value the test previously did. (Since our SPI provider has defined preferred Japanese Locale name values.) This change also refactors the test to use JUnit. The test now runs around ~20 times faster.
> 
> Most of the diff is attributed to separating the method source from the test itself. But notice the filtering of `Locale`s done on `availloc` and `jreImplLoc`.

Justin Lu has updated the pull request incrementally with one additional commit since the last revision:

  Naoto review - update comment, isEmpty()

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/25009/files
  - new: https://git.openjdk.org/jdk/pull/25009/files/c3e77cde..579c1ebe

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=25009&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=25009&range=00-01

  Stats: 6 lines in 1 file changed: 1 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/25009.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25009/head:pull/25009

PR: https://git.openjdk.org/jdk/pull/25009