Run same tests for layout-2020 and layout-2013 (#30092)

Most tests were only being run for layout-2013, not for layout-2020.
This wasn't great since layout-2020 is now the default.

So this patch unifies the lists of included tests for both layouts.
For layout-2013 this implies adding css/css-content/, css/css-logical/
and css/css-masking/clip/.
For layout-2020 this implies adding several additional css tests, and
also tests like dom/, js/, html/, etc.

tests/wpt/meta/domparsing/XMLSerializer-serializeToString.html.ini

@@ -0,0 +1,63 @@
+[XMLSerializer-serializeToString.html]
+  [check XMLSerializer.serializeToString method could parsing document to string]
+    expected: FAIL
+
+  [Check if the default namespace is correctly reset.]
+    expected: FAIL
+
+  [Check if there is no redundant empty namespace declaration.]
+    expected: FAIL
+
+  [Check if redundant xmlns="..." is dropped.]
+    expected: FAIL
+
+  [Check if inconsistent xmlns="..." is dropped.]
+    expected: FAIL
+
+  [Check if an attribute with namespace and no prefix is serialized with the nearest-declared prefix]
+    expected: FAIL
+
+  [Check if an attribute with namespace and no prefix is serialized with the nearest-declared prefix even if the prefix is assigned to another namespace.]
+    expected: FAIL
+
+  [Check if the prefix of an attribute is replaced with another existing prefix mapped to the same namespace URI.]
+    expected: FAIL
+
+  [Check if the prefix of an attribute is NOT preserved in a case where neither its prefix nor its namespace URI is not already used.]
+    expected: FAIL
+
+  [Check if the prefix of an attribute is replaced with a generated one in a case where the prefix is already mapped to a different namespace URI.]
+    expected: FAIL
+
+  [check XMLSerializer.serializeToString escapes attribute values for roundtripping]
+    expected: FAIL
+
+  [Check if attribute serialization takes into account of following xmlns:* attributes]
+    expected: FAIL
+
+  [Check if attribute serialization takes into account of the same prefix declared in an ancestor element]
+    expected: FAIL
+
+  [Check if start tag serialization drops element prefix if the namespace is same as inherited default namespace.]
+    expected: FAIL
+
+  [Check if start tag serialization finds an appropriate prefix.]
+    expected: FAIL
+
+  [Check if start tag serialization takes into account of its xmlns:* attributes]
+    expected: FAIL
+
+  [Check if start tag serialization applied the original prefix even if it is declared in an ancestor element.]
+    expected: FAIL
+
+  [Check if start tag serialization does NOT apply the default namespace if its namespace is declared in an ancestor.]
+    expected: FAIL
+
+  [Check if generated prefixes match to "ns${index}".]
+    expected: FAIL
+
+  [Check if "ns1" is generated even if the element already has xmlns:ns1.]
+    expected: FAIL
+
+  [Check if no special handling for XLink namespace unlike HTML serializer.]
+    expected: FAIL