We are moving to simpletest/simpletest. The composer packages have also been updated.
To get your project up-to-date with these changes, it is recommended to execute following command in your project's directory.
find ./ -type f -exec sed -i 's:vierbergenlars/simpletest:simpletest/simpletest:g' {} \;
This will find and replace vierbergenlars/simpletest with simpletest/simpletest everywhere.