Improve test executor readability

[comane]

This PR does:

• adds a return to the execute_parallel method in resource builder returning the client object (with close() method)

• renames some of the toy functions used to generate a toy DAG with diamond structure.

• adds some docstrings as well as remove some unused toy functions whose names were: n, o, p

Note: client.close is needed by test_executor and test_builder. Otherwise if test_builder is pytested runned before the test_executor, test_executor will be stuck because the client has not been closed appropriately

[scarlehoff]

Thanks. I don't know enough RE to offer a more meaningful review though.

[RoyStegeman]

Thanks. Unfortunately, I have the same limitation as @scarlehoff. I suppose you're the master of reportengine now ;)

However, with the exception of the first point the changes are just cosmetic (and I agree it's clearer now), so this should be good to merge.