Auto merge of #16752 - jdm:css-parse-error, r=SimonSapin

Report more informative CSS errors

This requires https://github.com/servo/rust-cssparser/pull/143 for the final commit. There's no better way to split that work up, unfortunately, and it's extremely easy to bitrot. I would appreciate if we could expedite reviewing this work.

This is the work necessary to enable https://bugzilla.mozilla.org/show_bug.cgi?id=1352669. It makes sense to merge it separately because it's so much effort to keep it up to date with the ongoing Stylo work.

---
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [x] There are tests for these changes

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python/servo/testing_commands.py

@@ -232,6 +232,15 @@ class MachCommands(CommandBase):
                 test_patterns.append(test)
 
         in_crate_packages = []
+
+        # Since the selectors tests have no corresponding selectors_tests crate in tests/unit,
+        # we need to treat them separately from those that do.
+        try:
+            packages.remove('selectors')
+            in_crate_packages += ["selectors"]
+        except KeyError:
+            pass
+
         if not packages:
             packages = set(os.listdir(path.join(self.context.topdir, "tests", "unit"))) - set(['.DS_Store'])
             in_crate_packages += ["selectors"]