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

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?