Auto merge of #15183 - servo:font-family, r=nox

Fix parsing and serialization of font-family

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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #15059 (github issue number if applicable).

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tests/wpt/mozilla/meta/MANIFEST.json

@@ -6788,6 +6788,12 @@
             "url": "/_mozilla/css/offset_properties_inline.html"
           }
         ],
+        "css/test_font_family_parsing.html": [
+          {
+            "path": "css/test_font_family_parsing.html",
+            "url": "/_mozilla/css/test_font_family_parsing.html"
+          }
+        ],
         "css/test_variable_legal_values.html": [
           {
             "path": "css/test_variable_legal_values.html",