Run tests with isolated context

[abhinayagarwal]

This is a WIP PR

[abhinayagarwal]

Findings from TestFX:

• Same Stage is used in every Test. This instance comes from PrimaryStageApplication
• PrimaryStageApplication.class is also finally passed to Application.launch method.
• In the tests, instances are as follows:
  • Always same: org.testfx.api.FxToolkitContext, javafx.stage.Stage
  • Always different: org.testfx.toolkit.impl.ToolkitServiceImpl, org.testfx.framework.junit5.ApplicationAdapter
• ToolkitServiceImpl.setupApplication creates a new instance of application (ApplicationAdapter) but the stage is created from Supplier stageSupplier and returns the following from FxToolkitContext:

private final CompletableFuture<Stage> primaryStageFuture = * PrimaryStageApplication.PRIMARY_STAGE_FUTURE; 

• GluonFX: Since AppManager is singleton, its instance is also same across all tests, which affects how we show and hide layers across different tests
  With IsolateContext , we have different instances of FxToolkitContext between test classes but the instance of Stage and AppManager still remains the same, not giving the effect which we initially wanted.