Tests: have test_object active by default in regression test files

When opening Geometry Nodes regression test files I always find a bit annoying
that the `expected_object` and not the `test_object` is active. That's because
only the `test_object` contains the modifier or node tree that is being tested.
Therefore, usually my first step when opening these files is to select the
`test_object` first.

This patch changes it so that when mesh tests are updated, the `test_object` is
made active in the end before the file is saved again.

Pull Request: https://projects.blender.org/blender/blender/pulls/139088

tests/python/modules/mesh_test.py

@@ -206,6 +206,8 @@ class MeshTest(ABC):
                 self.expected_object = objects[self.exp_object_name]
             else:
                 self.create_expected_object()
+                self.activate_test_object()
+                bpy.ops.wm.save_as_mainfile(filepath=bpy.data.filepath)
         else:
             self.expected_object = objects[self.exp_object_name]
 
@@ -221,7 +223,6 @@ class MeshTest(ABC):
         self.expected_object.name = self.exp_object_name
         x, y, z = self.test_object.location
         self.expected_object.location = (x, y + 10, z)
-        bpy.ops.wm.save_as_mainfile(filepath=bpy.data.filepath)
 
     def create_evaluated_object(self):
         """
@@ -239,6 +240,14 @@ class MeshTest(ABC):
         self.evaluated_object = bpy.context.active_object
         self.evaluated_object.name = "evaluated_object"
 
+    # Test files are less confusing when the test object is active initially instead of
+    # the expected object. That's because the test object has the modifier/node tree that
+    # is being tested.
+    def activate_test_object(self):
+        bpy.ops.object.select_all(action="DESELECT")
+        self.test_object.select_set(True)
+        bpy.context.view_layer.objects.active = self.test_object
+
     @staticmethod
     def _print_result(result):
         """
@@ -378,6 +387,8 @@ class MeshTest(ABC):
         self.evaluated_object.name = expected_object_name
         self.do_selection(self.evaluated_object.data, "VERT", evaluated_selection, False)
 
+        self.activate_test_object()
+
         # Save file.
         bpy.ops.wm.save_as_mainfile(filepath=bpy.data.filepath)
         self.test_updated_counter += 1