CI improvement: remove text and add a CI test or at least the print of the output of the objective value of the CI

[davide-f]

Describe the feature you'd like to see

I recommend to:

• add at least the print of the objective value of the optimization in the CI
• revise the CI to avoid the long text printing. That is due to (a) deprecation warning and (b) unnecessary printing (some pypsa-earth option may be exploited)

[GbotemiB]

Hi @davide-f,

Currently, the objective value is logged to a file in the logs directory. I think adding a print statement in the solve_network script should solve printing the output of the objective value.