test: adjust xpath gc test to libxml2's max recursion depth

See upstream libxml2 commit 6f1470a where the recursion limit is
hardcoded to 5000 stack frames.

Author: flavorjones

test/xml/test_xpath.rb

@@ -340,7 +340,7 @@ def name_equals(nodeset, name, *args)
           # long list of long arguments, to apply GC pressure during
           # ruby_funcall argument marshalling
           xpath = ["//tool[name_equals(.,'hammer'"]
-          1000.times { xpath << "'unused argument #{'x' * 1000}'" }
+          500.times { xpath << "'unused argument #{'x' * 1000}'" }
           xpath << "'unused argument')]"
           xpath = xpath.join(',')