Add missing tests for HelpWindow behaviour

[yamidark]

Refer to #850.

Due to an issue with JavaFX Monocle, where multiple stages can be in
focus when tests are run in headless mode, we are unable to test for
checking if HelpWindow comes into focus if we call
MainWindow#handleHelp() while the HelpWindow is open at the moment.

Tests for this behaviour should be added in once the issue in JavaFX Monocle has been resolved.