[IMP] test_themes: add test to detect accidental cross-theme references

This commit adds a test to ensure that assets and images referenced from
a theme do actually belong to that theme.

task-4146886

Part-of: odoo/design-themes#889
Signed-off-by: Arthur Detroux (ard) <ard@odoo.com>

test_themes/tests/__init__.py

@@ -6,6 +6,7 @@
 
 from . import test_crawl
 from . import test_new_page_templates
+from . import test_theme_scope
 from . import test_theme_upgrade
 # This test should be last.
 from . import test_theme_standalone