TMPDIR in test is not universally defined

[shadowspawn]

Using TMPDIR for /tmp, but only defined on macOS. Working by chance in docker containers.

See #8 (comment)

[shadowspawn]

optional TMPDIR is expected, but fallback to /tmp if not defined:

• https://superuser.com/questions/332610/where-is-the-temporary-directory-in-linux
• https://stackoverflow.com/questions/8087805/how-to-get-system-or-user-temp-folder-in-unix-and-windows
• https://en.wikipedia.org/wiki/Temporary_folder

[shadowspawn]

Fixed on develop for next release