Test coverage percentage needs to be increased

[ntorresd]

The test coverage of the development branch is currently around 85%. Our goal is to raise it above 90% before the next release and CRAN submission.

• Unit test is missing for simulate_serosurvey_general_model() #233
• Unit test missing for add_age_group_to_serosurvey() #236
• Increase test coverage of plot functions to 100% #240
• Unit test is missing for functions in summarise_seromodel.R #237