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

javafx.base unit test LocalDateTimeStringConverterTest changes default locale to US.
While it is needed for this test, setting default locale changes the global state of VM default. A cleaner approach would be -
1) Save the current default locale before this test is run
2) Set the default locale to US as this test needs it
3) Restore the default locale to the one saved in step 1 after all the test cases in LocalDateTimeStringConverterTest are executed.

A similar fix was done in controls test - please see JDK-8264952.