test: repro namespace_scopes compaction issue

Co-Authored-By: Matt Valentine-House <matt@eightbitraptor.com>
Co-Authored-By: Peter Zhu <peter@peterzhu.ca>

test/test_compaction.rb

@@ -18,4 +18,22 @@
     # access the node wrappers and make sure they didn't move
     doc.root.children.each(&:inspect)
   end
+
+  it "namespace_scopes compact" do
+    skip unless GC.respond_to?(:verify_compaction_references)
+
+    doc = Nokogiri::XML(<<~EOF)
+      <root xmlns="http://example.com/root" xmlns:bar="http://example.com/bar">
+        <first/>
+        <second xmlns="http://example.com/child"/>
+        <third xmlns:foo="http://example.com/foo"/>
+      </root>
+    EOF
+
+    doc.at_xpath("//root:first", "root" => "http://example.com/root").namespace_scopes.inspect
+
+    GC.verify_compaction_references(double_heap: true, toward: :empty)
+
+    doc.at_xpath("//root:first", "root" => "http://example.com/root").namespace_scopes.inspect
+  end
 end