Tests: Add Multires Apply Base test

Adds a new file for testing the Multires Apply Base operator.

This is not included with the other modifier tests as it is a bit of an
exception - the modifier is not applied at the end for comparison
between the expected result and actual result.

Pull Request: https://projects.blender.org/blender/blender/pulls/141571

tests/python/CMakeLists.txt

@@ -493,6 +493,13 @@ if(TEST_SRC_DIR_EXISTS)
     --
     --testdir "${TEST_SRC_DIR}/constraints"
   )
+
+  add_blender_test(
+    multires
+    --python ${TEST_PYTHON_DIR}/bl_multires.py
+    --
+    --testdir "${TEST_SRC_DIR}/sculpting"
+  )
 endif()
 
 # ------------------------------------------------------------------------------

tests/python/bl_multires.py

@@ -0,0 +1,58 @@
+# SPDX-FileCopyrightText: 2025 Blender Authors
+#
+# SPDX-License-Identifier: GPL-2.0-or-later */
+
+__all__ = (
+    "main",
+)
+
+import unittest
+import sys
+import pathlib
+
+import bpy
+from mathutils import Vector
+
+"""
+blender -b --factory-startup --python tests/python/bl_object_modifier_multires.py -- --testdir tests/files/sculpting/
+"""
+
+args = None
+
+
+class ApplyBase(unittest.TestCase):
+    def setUp(self):
+        bpy.ops.wm.open_mainfile(filepath=str(args.testdir / "apply_base_monkey.blend"), load_ui=False)
+
+    def test_subdividing_cube_results_in_same_mesh(self):
+        bpy.ops.mesh.primitive_monkey_add()
+        new_cube = bpy.context.object
+
+        multires_mod = new_cube.modifiers.new("Multires", 'MULTIRES')
+        bpy.ops.object.multires_subdivide(modifier="Multires")
+        bpy.ops.object.multires_base_apply(modifier="Multires")
+        bpy.ops.object.modifier_remove(modifier="Multires")
+
+        expected_mesh = bpy.data.objects['Expected_Base_Mesh']
+        result = expected_mesh.data.unit_test_compare(mesh=new_cube.data)
+        self.assertEqual(result, 'Same')
+
+
+def main():
+    global args
+    import argparse
+
+    argv = [sys.argv[0]]
+    if '--' in sys.argv:
+        argv += sys.argv[sys.argv.index('--') + 1:]
+
+    parser = argparse.ArgumentParser()
+    parser.add_argument('--testdir', required=True, type=pathlib.Path)
+
+    args, remaining = parser.parse_known_args(argv)
+
+    unittest.main(argv=remaining)
+
+
+if __name__ == '__main__':
+    main()