Add test to see if all deprecated theme icons still exist

This will make it easier in the future if we deprecate icons again. And it's nice for now.

Contributes to issue CURA-8520.

tests/TestThemes.py

@@ -0,0 +1,33 @@
+# Copyright (c) 2021 Ultimaker B.V.
+# Cura is released under the terms of the LGPLv3 or higher.
+
+import json  # To parse the deprecated icons files.
+import os  # To find the theme folders.
+import pytest
+
+theme_base = os.path.join(os.path.split(__file__)[0], "..", "resources", "themes")
+theme_paths = [os.path.join(theme_base, theme_folder) for theme_folder in os.listdir(theme_base) if os.path.isdir(os.path.join(theme_base, theme_folder))]
+
+@pytest.mark.parametrize("theme_path", theme_paths)
+def test_deprecatedIconsExist(theme_path: str) -> None:
+    icons_folder = os.path.join(theme_path, "icons")
+    deprecated_icons_file = os.path.join(icons_folder, "deprecated_icons.json")
+    if not os.path.exists(deprecated_icons_file):
+        return  # No deprecated icons file, there is nothing to go wrong.
+
+    # Find out which icons exist in this theme file.
+    existing_icons = {}
+    for size in [subfolder for subfolder in os.listdir(icons_folder) if os.path.isdir(os.path.join(icons_folder, subfolder))]:
+        existing_icons[size] = set(os.path.splitext(fname)[0] for fname in os.listdir(os.path.join(icons_folder, size)))
+
+    with open(deprecated_icons_file) as f:
+        deprecated_icons = json.load(f)
+
+    for entry in deprecated_icons.values():
+        assert "new_icon" in entry  # For each deprecated icon we must know which icon replaced it.
+        new_icon = entry["new_icon"]
+        assert "size" in entry
+        size = entry["size"]
+
+        assert size in existing_icons  # The replacement icon must have a size that exists.
+        assert new_icon in existing_icons[size]  # The new icon must exist for that size.