Use joinpath when splicing up paths in test files

[tdivoll]

          String interpolation/concatenation works fine here, but people seem to say that the "correct" way to do this is to use `joinpath` when splicing up paths (see for example [this discussion](https://stackoverflow.com/questions/32070934/what-is-the-advantage-of-using-path-combine-over-concatenating-strings-with)).

Because many paths are being created with the same base directory, I would make a custom function to simplify this a bit. Something like this:

julia> test_data_dir = "foo"
"foo"

julia> jointdd(s) = joinpath(test_data_dir, s)
jointdd (generic function with 1 method)

julia> jointdd("bar")
"foo\\bar"

Then line 6, for example, would reduce to

segmentation_B_not_ice_mask = float64.(load(jointdd("matlab_I.png")))

Or even throw in load:

loadtdd(s) = load(joinpath(test_data_dir, s))

segmentation_B_not_ice_mask = float64.(loadtdd("matlab_I.png"))

Also, I think piping would probably work nicely for such use case:

loadtdd = jointdd |> load
segmentation_B_not_ice_mask = "matlab_I.png" |>loadtdd  |> x->float64.(x)

Totally up to you, though! It's only a suggestion. 😃

Originally posted by @cpaniaguam in #130 (comment)