Fix rake failure since `mrbtest_objs` isn't flattened.
Append generated test object to `@test_objs` of mrbgem spec.
Add method `custom_test_init?` to check whether mrbgem has custom test init function.
Merge pull request #2463 from take-cheeze/fix_rake

Fix rake failure since `mrbtest_objs` isn't flattened.