Add dummy as a known platform, and use it in tests

src/core/toga/platform.py

@@ -28,6 +28,9 @@ def get_platform_factory(factory=None):
     if factory is not None:
         return factory
 
+    if current_platform == 'dummy':
+        from toga_dummy import factory
+        return factory
     if current_platform == 'android':
         from toga_android import factory
         return factory

src/dummy/toga_dummy/utils.py

@@ -1,10 +1,13 @@
 import sys
 import unittest
 
+import toga.platform
 from travertino.declaration import BaseStyle
 from travertino.layout import BaseBox
 from travertino.size import BaseIntrinsicSize
 
+toga.platform.current_platform = 'dummy'
+
 
 def not_required(method_or_class):
     """ This decorator function is used to mark methods or classes