introduce debug check.

There are random failures:

> lib/rubygems/core_ext/kernel_require.rb:61:in `require':
> wrong number of arguments (given 1, expected 0) (ArgumentError)
http://ci.rvm.jp/results/trunk-jemalloc@silicon-docker/2275159

To check this failure, I added a small check code.

tool/lib/leakchecker.rb

@@ -18,10 +18,21 @@ class LeakChecker
       check_encodings(test_name),
       check_safe(test_name),
       check_verbose(test_name),
+      check_require_arity(test_name),
     ]
     GC.start if leaks.any?
   end
 
+  # to debug
+  def check_require_arity test_name
+    begin
+      require 'enumerator' # to check require's arity
+    rescue ArgumentError => e
+      STDERR.puts "#{e.message} on #{test_name}"
+      raise
+    end
+  end
+
   def check_safe test_name
     puts "#{test_name}: $SAFE == #{$SAFE}" unless $SAFE == 0
   end