execute tests in a subfolder of $TMPDIR

- using predictable names in /tmp for doing anything is bad for security
- trying to re-execute tests after one failed atempt may result in weird
  behaviour instead of working (or failing the very same way) tests

let's call mktemp -d, work in the directory it creates and remove
everything when done
download and build lib in an non-predictable temp folder
Merge pull request #183 from evgeni/proper-tmp-use

do not place tests in predictable temp folders