servo: Merge #19070 - Enable CSS tests (from jdm:csstests); r=metajack

Do not merge this; I'm looking into what it takes to stop running the test-css jobs.

Source-Repo: https://github.com/servo/servo
Source-Revision: 86b9e7d7d604e00cfd7ab63351d3221cd5cf872e

servo/python/tidy/servo_tidy/tidy.py

@@ -316,7 +316,7 @@ def check_flake8(file_name, contents):
 
 def check_lock(file_name, contents):
     def find_reverse_dependencies(name, content):
-        for package in itertools.chain([content["root"]], content["package"]):
+        for package in itertools.chain([content.get("root", [])], content["package"]):
             for dependency in package.get("dependencies", []):
                 if dependency.startswith("{} ".format(name)):
                     yield package["name"], dependency