test: reduce the number of rehearsal executions

It reduces test execution time.

test/test_document.rb

@@ -204,7 +204,7 @@ def test_xml_declaration_standalone
 
     def test_gt_linear_performance_attribute_value
       seq = [10000, 50000, 100000, 150000, 200000]
-      assert_linear_performance(seq) do |n|
+      assert_linear_performance(seq, rehearsal: 10) do |n|
         REXML::Document.new('<test testing="' + ">" * n + '"></test>')
       end
     end