You don't want this - `/tmp/securedrop_test` is the scratch directory used by the unit tests (see `config.py`). If you set your development install's `SECUREDROP_ROOT` to this, then it would get overwritten every time you ran the unit tests.

In fact, SECUREDROP_ROOT shouldn't be in `/tmp` at all. Otherwise it gets deleted on reboot (this is automatic on most OS). I'm going to open a pull request to remedy this. The new default location is `securedrop/securedrop/.securedrop`, which is gitignore'd.