UI: Add UI tests for opening editors

Editor initialization by dynamically constructing a grid layout,
iterating through all settable editor variants, applying editor-specific
modes/subtypes, and validating successful initialization of each editor
instance.

These tests already proved themselves useful, we already found and fixed
a number of issues during development.

Pull Request: https://projects.blender.org/blender/blender/pulls/158780

tests/python/ui_simulate/modules/ui_test_utils.py

@@ -205,3 +205,41 @@ def largest_area(screen):
         screen.areas,
         key=lambda a: a.width * a.height
     )
+
+
+def split_area(area, direction='VERTICAL', factor=0.5):
+    """
+    Split an area using Blender's screen split operator.
+
+    :param area: area to split.
+    :param direction: plit direction: 'VERTICAL' / 'HORIZONTAL'.
+    :param factor: Split ratio in normalized coordinates.
+        Default 0.5 creates an even split.
+    """
+
+    import bpy
+    with bpy.context.temp_override(area=area):
+
+        bpy.ops.screen.area_split(
+            direction=direction,
+            factor=factor,
+        )
+
+
+def build_grid(screen, target_count):
+    """
+    Dynamically construct a visible grid of areas.
+    """
+    while len(screen.areas) < target_count:
+        prev_count = len(screen.areas)
+        area = largest_area(screen)
+        direction = (
+            'VERTICAL'
+            if area.width >= area.height
+            else 'HORIZONTAL'
+        )
+
+        split_area(area, direction, 0.5)
+        yield
+        if len(screen.areas) == prev_count:
+            raise Exception(f"Area split did not increase area count (stuck at {prev_count}). ")