Support SceneTree and signal testing - Code edit unit tests

[Paulb23]

Builds on and used to test #50372

Related godotengine/godot-proposals/issues/1307

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Support SceneTree and signal testing, opening up for feedback.

Test code changes can be added separately to #50372 but I figured it made sense to have an example with it, so included it here.

Added a doctest reporter GodotTestCaseListener. When doctest runs the test_case_start callback (not subcase) it will check if the case name contains, [SceneTree]; if found will create all the required engine parts.

It will then cleanup that code on the test_case_end callback.

A key part of #50372 is user input and signals.

To handle this a macro SEND_GUI_ACTION(object, action_name) has been added, this will call the _gui_input method on the object with a given action, i.e SEND_GUI_ACTION(code_edit, "ui_text_newline"). This requires a MessageQueue and InputMap.

For signals a test singleton SignalWatcher as been added, when a signal is connected, the SignalWatcher between checks, will capture every time it is emitted with it's args.

The following macros can be use to check signals:

• SIGNAL_WATCH(*object, "signal_name") to start watching.
• SIGNAL_UNWATCH(*object, "signal_name") to stop watching.
• SIGNAL_CHECK("signal_name"), Vector<Vector<Variant>>), to check the arguments of all fired signals.
  • The outer vector is each fired signal, the inner vector the list of arguments for that signal. Order does matter.
• SIGNAL_CHECK_FALSE("signal_name") to check if a signal was not fired.
• SIGNAL_DISCARD("signal_name") to discard records all of the given signal

Finally, two minor bugfixs where the input map and display server where not nulling it's singleton var, causing it to print a warning every time it was re-setup. In addition rasterizer_dummy.h was returning true when it has not freed the memory causing a minor memory leak.

[Paulb23]

Reabsed following #51983 which removed the ability to call "_gui_input". As a result the SEND_GUI_ACTION macro now makes a ordinary c++ call to gui_input, meaning it has to be made public.

[Paulb23]

Fixed a crash after #51296 which assumed there would be an AudioServer, added a check for the singleton in Viewport before we call it.

[akien-mga]

Thanks!