Enable Shadow DOM for tests (#34299)

* Enable shadowdom implementation in all tests.

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

* Update test expectations.

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

---------

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

tests/wpt/meta/shadow-dom/imperative-slot-api.html.ini

@@ -0,0 +1,45 @@
+[imperative-slot-api.html]
+  [attachShadow can take slotAssignment parameter.]
+    expected: FAIL
+
+  [slot.attach() should take variadic not sequence.]
+    expected: FAIL
+
+  [Imperative slot API can assign nodes in manual slot assignment.]
+    expected: FAIL
+
+  [Order of slottables is preserved in manual slot assignment.]
+    expected: FAIL
+
+  [Previously assigned slottable is moved to new slot when it's reassigned.]
+    expected: FAIL
+
+  [Order and assignment of nodes are preserved during multiple assignment in a row.]
+    expected: FAIL
+
+  [Assigning invalid nodes should be allowed.]
+    expected: FAIL
+
+  [Moving a slot to a new host, the slot loses its previously assigned slottables.]
+    expected: FAIL
+
+  [Moving a slot's tree order position within a shadow host has no impact on its assigned slottables.]
+    expected: FAIL
+
+  [Appending slottable to different host, it loses slot assignment. It can be re-assigned within a new host.]
+    expected: FAIL
+
+  [Previously assigned node should not be assigned if slot moved to a new shadow root. The node is re-assigned when moved back.]
+    expected: FAIL
+
+  [Assignment with the same node in parameters should be ignored, first one wins.]
+    expected: FAIL
+
+  [Removing a slot from DOM resets its slottable's slot assignment.]
+    expected: FAIL
+
+  [Nodes can be assigned even if slots or nodes aren't in the same tree.]
+    expected: FAIL
+
+  [Removing a node from the document does not break manually assigned slot linkage.]
+    expected: FAIL