Implement shiny app tests with 'shinytest2'

[mbojan]

To address, among other things, #63, but primarily to catch other potential problems implement some tests using shinytest2 package.

[mbojan]

Implemented a rather rudimentary test that checks if loading flobusiness works and the app looks as it should after that -- essentially it compares a specially made screenshots. Further tests can be added with shinytest2::record_test().