test: use double quote for string literal

Author: kou

test/parse/test_entity_declaration.rb

@@ -521,7 +521,7 @@ def test_empty
     def test_linear_performance_gt
       seq = [10000, 50000, 100000, 150000, 200000]
       assert_linear_performance(seq, rehearsal: 10) do |n|
-        REXML::Document.new('<!DOCTYPE rubynet [<!ENTITY rbconfig.ruby_version "' + '>' * n + '">]>')
+        REXML::Document.new("<!DOCTYPE rubynet [<!ENTITY rbconfig.ruby_version \"" + ">" * n + "\">]>")
       end
     end
   end