[R unit tests] Fix path and package installation

[gsvidaurre]

Fix the following:

1. The path for unit testing needs to be generalized, ideally in the main script to run all unit tests
2. The code to install packages if they're not already installed should updated to:

is_installed <- function(p) is.element(p, installed.packages()[,1])

invisible(lapply(1:length(X), function(x){
if(!is_installed(X[x])){
install.packages(X[x], repos = "http://lib.stat.cmu.edu/R/CRAN")
}
}))