Auto merge of #9333 - servo:list-properties, r=larsbergstrom

Add a script to list all CSS properties parsed by Servo.

I’ve been asked for that list by two different people this week :)

r? @larsbergstrom

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python/servo/testing_commands.py

@@ -15,6 +15,7 @@ import sys
 import os
 import os.path as path
 import subprocess
+import json
 from collections import OrderedDict
 from time import time
 
@@ -158,6 +159,14 @@ class MachCommands(CommandBase):
     @CommandArgument('test_name', nargs=argparse.REMAINDER,
                      help="Only run tests that match this pattern or file path")
     def test_unit(self, test_name=None, package=None):
+        properties = json.loads(subprocess.check_output([
+            sys.executable,
+            path.join(self.context.topdir, "components", "style", "list_properties.py")
+        ]))
+        assert len(properties) >= 100
+        assert "margin-top" in properties
+        assert "margin" in properties
+
         if test_name is None:
             test_name = []