Allow setting custom temp directory

[rask]

I can't see any configurability for setting the temporary directory where PT_ and CV_ files are written.

Could that be exposed, as sometimes PHP can read get_sys_tmp_dir but cannot write to it?

[Slamdunk]

This would be a good improvement. Since we would like to edit phpunit configuration file, it should be a separate option in the command. Would you like to propose a PR for this?