RFR: 8264952: [TestBug] Controls unit tests - ControlTest and SpinnerTest fail for non US Locale

[Ajit Ghaisas]

This PR fixes controls unit test failures that are observed with German locale.

Fix :
Code added to use US locale while running SpinnerTest and DatePicker test & restore locale to default after these tests are executed.

test.javafx.scene.control.ControlTest.testRT18097 fails for DatePicker control. Fixing DatePickerTest fixes this. This reveals the fact that our unit tests run in a single VM and any test that changes the Locale setting should restore it to the default at the end.

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Commit messages:
 - fix_controls_test_locale

Changes: https://git.openjdk.java.net/jfx/pull/476/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=476&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264952
  Stats: 29 lines in 2 files changed: 25 ins; 1 del; 3 mod
  Patch: https://git.openjdk.java.net/jfx/pull/476.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/476/head:pull/476

PR: https://git.openjdk.java.net/jfx/pull/476