fix(test): Each test class that uses JFX must init the JFX toolkit

[arnobl]

When executing the class JavaFxObservableTest alone several tests fail: the JFX tooltkit is not initialised.
The test class JavaFxSchedulerTest has a method initJFX that initialises the JFX toolkit. Yet, if JavaFxObservableTest is executed before this class, the tests will fail (which appends sometimes with gradle build).

Another change: if the toolkit has been already initialised by another test class, an annoying exception will be raised (IllegalStateException). Need to try/catch to ignore this exception.