Fix dx download truncation test on OS X
Fix dx download truncation test on OS X

Use truncate from Python instead of assuming that the CLI / gnu
coreutils utility is there.

johnwallace123/dx-toolkit,johnwallace123/dx-toolkit,jhuttner/dx-toolkit,dnanexus/dx-toolkit,dnanexus/dx-toolkit,johnwallace123/dx-toolkit,dnanexus/dx-toolkit,dnanexus/dx-toolkit,johnwallace123/dx-toolkit,jhuttner/dx-toolkit,dnanexus/dx-toolkit,jhuttner/dx-toolkit,jhuttner/dx-toolkit,jhuttner/dx-toolkit,dnanexus/dx-toolkit,jhuttner/dx-toolkit,jhuttner/dx-toolkit,johnwallace123/dx-toolkit,johnwallace123/dx-toolkit,dnanexus/dx-toolkit,jhuttner/dx-toolkit,dnanexus/dx-toolkit,johnwallace123/dx-toolkit,johnwallace123/dx-toolkit