The problem is that, somehow, Project.toml in the - test - directory didn’t include BusinessDays as a dependency, although it was included as a dependency in DayCounts' Project.toml. So after doing the following the problem was solved:
julia> cd("test")
(DayCounts) pkg> activate .
Activating environment at `~/Documents/GitHub/DayCounts.jl/test/Project.toml`
(test) pkg> add BusinessDays
Thank you very much for your help diagnosing the problem.