Improve test coverage

[LucieContamin]

Currently, we have a test coverage of:

rama Coverage: 41.75%
R/tidyverse.R: 10.04%
R/base.R: 13.70%
R/utils.R: 33.63%
R/create_output_dir.R: 53.85%
R/map_experiment.R: 56.25%
R/output_dir.R: 57.14%
R/plot_params.R: 60.71%
R/fill_experiment.R: 66.67%
R/name.R: 66.67%
R/obs_rates.R: 66.67%
R/parameters.R: 66.67%
R/zzz.R: 71.62%
R/model.R: 72.22%
R/experiment.R: 74.60%
R/repl.R: 75.00%
R/is.experiment.R: 77.78%
R/show_experiment.R: 84.21%
R/as_experiment.R: 85.71%
R/call_gama.R: 90.00%
R/load_experiment.R: 92.11%
R/run_experiment.R: 95.92%
R/save_to_gama.R: 96.61%
R/fullfact.R: 100.00%
R/get_names.R: 100.00%