Use proper relative references for submodules to allow using mirror repositories

Currently submodules are improperly defined which makes it troublesome to use mirror repositories such as https://github.com/openhive-network/hivemind

hive@lab:~/hivemind$ git submodule update --init --recursive
Submodule 'tests/tests_api' (https://github.com/hive/tests_api.git) registered for path 'tests/tests_api'
Cloning into '/home/hive/hivemind/tests/tests_api'...

(i.e. it should look for https://github.com/openhive-network/tests_api.git)