Add test cases for antigen-selfupdate

This is the test cases for #65. The test procedure is listed as following:

1. Create a repository as host
2. Clone host repository as normal and submodule repository
3. Update host repository
4. use `antigen-selfupdate` to update normal and submodule repository

I am not sure if this is okay to test `antigen-selfupdate`. Please help to review it, thanks.