Improve the current_cases fixture: add the full case id

[smarie]

With the current_cases fixture one can access all information about the case currently being used for a given parameter.

However if the case is itself parametrized, the id available in current_cases[argname].id only corresponds to the "base" of the case, and is therefore not unique. This is a problem if we wish to use this id to fill a results table in a benchmark for example.

Proposal: add a .full_id argument that should return the part of the pytest id that corresponds to this case, therefore including the parameters and also the required fixture parameters (see #216).

Basically an id representing the current callspec of the case.