Add unit test for visualisation functionalities

[ntorresd]

The visualisation functions in R/plot_seromodel.R need dedicated unit tests. We should write tests for the following functions (see #210):

• plot_seromodel()
  • plot_summary()
  • plot_seroprevalence_estimates()
  • plot_serosurvey()
  • plot_foi_estimates()
  • plot_rhats()

[ntorresd]

Closed by #223