I don't think you can do UNC paths for the ProcessBuilder, but it doesn't really matter in any case.
To run a .bat file, you need to run a windows command shell and have that execute the .bat file, and the command shell doesn't support UNC paths... The way around it is to run your command like this:
cmd.exe /C "pushd \\testserver\someFolderName && test.bat && popd"
Essentially, you're telling the cmd prompt to mount your remote folder as a temporary drive (pushd \testserver\someFolderName), run test.bat and then unmount the temporary drive (popd).