- introduced `test/utils/test_lib.inc.sh` to simplify usage of
sandboxed node/client in the testsuite
- it reuses code from `./script/{node,client}_lib.inc.sh`
- use `wait_for_the_node_to_be_ready` to properly wait for the node to
be launched rather to use a fexed delay
- `test_multinode.sh` now launch 8 nodes.
