Fix 'make test' fail to build on Windows(MSVC)

tasks/mrbgems_test.rake

@@ -115,7 +115,8 @@ MRuby.each_target do
     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"
+  no_mrb_open_test_lib = no_mrb_open_test.ext(exts.object)
+  file no_mrb_open_test_lib => "#{no_mrb_open_test}.c"
   file "#{no_mrb_open_test}.c" => no_mrb_open_test_rbfiles do |t|
     open(t.name, 'w') do |f|
       f.puts %Q[/*]

test/mrbtest.rake

@@ -16,7 +16,7 @@ MRuby.each_target do
   file mrbtest_lib => [mlib, ass_lib, gems.map(&:test_objs), gem_test_files].flatten do |t|
     archiver.run t.name, t.prerequisites
   end
-  file mrbtest_lib => "#{build_dir}/test/no_mrb_open_test.o"
+  file mrbtest_lib => "#{build_dir}/test/no_mrb_open_test.c".ext(exts.object)
 
   unless build_mrbtest_lib_only?
     driver_obj = objfile("#{current_build_dir}/driver")