testing strategy is not portable to windows

[jameslamb]

As of #108 , we added a bunch of new stuff to make our code testable on CRAN.

To make forward progress on the issues faced in that PR, we made a choice to use an approach that is Linux-specific (calling which -a R).

Addressing this issue means replacing this: https://github.com/UptakeOpenSource/pkgnet/blob/master/R/testing_utils.R#L46 or wrapping it in some kind of if-else statement of the form

if (ON UNIX_ALIKE){
    R_LOC <- system("which -a R", intern = TRUE)
    R_LOC <- R_LOC[!grepl("R_check_bin", R_LOC)][1]
} else {
    other_stuff()
}

[jameslamb]

more proof from devtools::build_win():

https://win-builder.r-project.org/yHIi38dYz5RI/00check.log