RFR: 8265828: [TestBug] Save and restore the default Locale in javafx.base unit test LocalDateTimeStringConverterTest [v2]

[Lukasz Kostyra]

> The change moves Locale setting in the test to `@BeforeClass` and `@AfterClass` calls. `@BeforeClass` method call stores current default VM locale and applies Locale.US, while `@AfterClass` method restores old VM locale after all tests are completed.
> 
> I tested it both on Mac and Windows, in both cases Locale is changed, restored properly and tests pass.

Lukasz Kostyra has updated the pull request incrementally with two additional commits since the last revision:

 - Refactor remaining LocalStringConverter tests
   
   Treatment done in this commit is similar to the previous change.
 - LocalDateTimeStringConverterTest: Refactor test to properly utilize Locale
   
   * Locale initialization was moved to @BeforeClass method.
   * DateTimeFormatter objects are allocated after Locale initialization
   * Since LocalDateTimeStringConverter depends on DateTimeFormatter and on VM's Locale,
     creation of it was moved to @Before method.

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

Changes:
  - all: https://git.openjdk.org/jfx/pull/954/files
  - new: https://git.openjdk.org/jfx/pull/954/files/1750b241..35b78479

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=954&range=01
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=954&range=00-01

  Stats: 254 lines in 3 files changed: 193 ins; 18 del; 43 mod
  Patch: https://git.openjdk.org/jfx/pull/954.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/954/head:pull/954

PR: https://git.openjdk.org/jfx/pull/954