Make benchmark exapmles as optional

Merged: https://github.com/ruby/ruby/pull/12537

spec/syntax_suggest/integration/syntax_suggest_spec.rb

@@ -235,5 +235,5 @@ module SyntaxSuggest
         end_is_missing_here
       EOM
     end
-  end
+  end if defined?(::Benchmark)
 end

spec/syntax_suggest/spec_helper.rb

@@ -3,7 +3,10 @@
 require "bundler/setup"
 require "syntax_suggest/api"
 
-require "benchmark"
+begin
+  require "benchmark"
+rescue LoadError
+end
 require "tempfile"
 
 RSpec.configure do |config|