!!! WIP !!! use recursive tests

Still missing are:
- t3426-rebase-submodule.sh
- t3512-cherry-pick-submodule.sh
- t3513-revert-submodule.sh
- t3906-stash-submodule.sh
- t4137-apply-submodule.sh
- t4255-am-submodule.sh
- t5572-pull-submodule.sh