run enabled gem's bintest only

tasks/mruby_build.rake

@@ -194,7 +194,8 @@ module MRuby
     end
 
     def run_bintest
-      sh "ruby test/bintest.rb"
+      targets = @gems.select { |v| Dir.exists? "#{v.dir}/bintest" }.map { |v| filename v.dir }
+      sh "ruby test/bintest.rb #{targets.join ' '}"
     end
 
     def print_build_summary

test/bintest.rb

@@ -1,8 +1,10 @@
 $:.unshift File.dirname(File.dirname(File.expand_path(__FILE__)))
 require 'test/assert.rb'
 
-Dir['mrbgems/**/bintest/*.rb'].each do |file|
-  load file
+ARGV.each do |gem|
+  Dir["#{gem}/bintest/*.rb"].each do |file|
+    load file
+  end
 end
 
 load 'test/report.rb'