Add verbose flag to cross test runner too.

lib/mruby/build.rb

@@ -344,6 +344,7 @@ EOS
     end
 
     def run_test
+      @test_runner.runner_options << ' -v' if $verbose
       mrbtest = exefile("#{build_dir}/bin/mrbtest")
       if (@test_runner.command == nil)
         puts "You should run #{mrbtest} on target device."