Use tempname() instead of custom file path in test/pkg.jl

[nalimilan]

Fixes calling Base.runtests("pkg") when Julia is installed in a
prefix where typical user does not have write access.

Please just confirm there wasn't a good reason not to use tempname. BTW, I can also update the docs for that function to say that it "Generate a unique temporary file or directory name", not just a "temporary filename". Or even better, a "temporary file or directory path". Right?

[ViralBShah]

Cc: @JuliaBackports

[tkelman]

backported in 84f6a45

[tkelman]

I think I'm going to have to revert this on both branches, will test locally first. Since tempname() creates a file on Windows (#9053) it causes Pkg.init() to fail, when a file exists in a location that it's trying to create a directory.