Make tests work when not installing with -e

[MichaelClerx]

Possibly solved by #35

At the moment, the instructions don't work if you install without -e.
This is because the test data is loaded from within the installed package. When using -e this is the same as the source directory (which is where the instructions tell you to place the test_data).

At the same time, (and when not using -e) you probably don't want the test data stored twice (in source and in install) so good solution might be for the package to auto-download it during or after install