Add dozens of new integration tests to the GDScript test suite

This also ignores `.out` files in the file format static checks.

modules/gdscript/tests/scripts/parser/warnings/void_assignment.gd

@@ -0,0 +1,6 @@
+func i_return_void() -> void:
+	return
+
+
+func test():
+	var __ = i_return_void()