default.mspec: limit cores

* spec/default.mspec (MSpecScript::JobServer#cores): limit max
  number of processors, not to acquire tokens more than necessary.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@59103 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

spec/mspec/lib/mspec/commands/mspec.rb

@@ -98,7 +98,7 @@ class MSpecMain < MSpecScript
     end
 
     output_files = []
-    processes = [cores, @files.size].min
+    processes = cores(@files.size)
     children = processes.times.map { |i|
       name = tmp "mspec-multi-#{i}"
       output_files << name

spec/mspec/lib/mspec/utils/script.rb

@@ -234,9 +234,9 @@ class MSpecScript
     files patterns
   end
 
-  def cores
+  def cores(max = 1)
     require 'etc'
-    Etc.nprocessors
+    [Etc.nprocessors, max].min
   end
 
   def setup_env