test: update tests to reflect upstream libxml2 text node coalescing

A text node coalescing bug was fixed in GNOME/libxml2@4ccd3eb8, and
although Nokogiri handles this fine from a memory perspective, the
tests need to be updated to reflect the new behavior.

Author: flavorjones

test/xml/test_node_reparenting.rb

@@ -623,7 +623,11 @@ def coerce(data)
               assert_equal "after", after.content
               refute_nil after.parent, "unrelated node should not be affected"
 
-              assert_equal "before", before.content
+              if Nokogiri.uses_libxml?(">= 2.13.0")
+                assert_equal "beforex", before.content # coalescing fixed in gnome/libxml2@4ccd3eb8
+              else
+                assert_equal "before", before.content
+              end
               refute_nil before.parent, "no need to reparent"
             end
           end
@@ -662,7 +666,11 @@ def coerce(data)
               assert_equal "before", before.content
               refute_nil before.parent, "unrelated node should not be affected"
 
-              assert_equal "after", after.content
+              if Nokogiri.uses_libxml?(">= 2.13.0")
+                assert_equal "xafter", after.content # coalescing fixed in gnome/libxml2@4ccd3eb8
+              else
+                assert_equal "after", after.content
+              end
               refute_nil after.parent
             end
           end