Make test_editable_install suitable for (repeated) local testing
Make test_editable_install suitable for (repeated) local testing

It turns out that when running `tox' multiple times, the test of editable
installations would fail on all but the first run. Of course it works fine
on Travis CI which always starts from a clean slate, but this quickly gets
annoying when running the test suite locally :-).

paylogic/pip-accel,paylogic/pip-accel,theyoprst/pip-accel,matysek/pip-accel,pombredanne/pip-accel,pombredanne/pip-accel,theyoprst/pip-accel,matysek/pip-accel