Fix build fail for 'make test' with no mrbgems

tasks/mrbgems_test.rake

@@ -111,6 +111,10 @@ MRuby.each_target do
   no_mrb_open_test_rbfiles = no_mrb_open_test_gem.reduce([]) { |res, v|
     res += v.test_rbfiles
   }
+  if no_mrb_open_test_rbfiles.empty?
+    no_mrb_open_test_rbfiles << "#{MRUBY_ROOT}/test/no_mrb_open_test_dummy.rb"
+  end
+  
   file "#{no_mrb_open_test}.o" => "#{no_mrb_open_test}.c"
   file "#{no_mrb_open_test}.c" => no_mrb_open_test_rbfiles do |t|
     open(t.name, 'w') do |f|

test/no_mrb_open_test_dummy.rb

@@ -0,0 +1,2 @@
+#dummy
+