benchmark/driver: avoid large alloc in driver process

* benchmark/driver: avoid large alloc in driver process
  [ruby-core:59869] [Bug #9430]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44772 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

benchmark/driver.rb

@@ -242,7 +242,7 @@ class BenchmarkDriver
     cmd = "#{executable} #{@ruby_arg} #{file}"
 
     m = Benchmark.measure{
-      `#{cmd}`
+      system(cmd, out: File::NULL)
     }
 
     if $? != 0