Adds additional tests for the interface. Creates a new base test class
GodotXtermTest that adds some additional assert methods. Tests
inheriting from this should override the got_described_class() method.
Add instance of the described class named 'subject' will be created and
added to the scene tree before each test.
Disconnects the "gui_input" signal as the _gui_input() override is
working as expected now. Having this signal connected was causing it to
be called twice for every input.
