Fix test on systems without /usr/bin/python

[scarabeusiv]

We use sys.executable to always load with interpreter the tests were executed with.

[coveralls]

Coverage increased (+1.02%) to 30.171% when pulling 9d78f68a74a993d88c94f6e037e1279aee620497 on scarabeusiv:patch-1 into 6788d15b017d7d6a3cf1834c9b098db2719762a9 on tomerfiliba:master.

[henryiii]

Thanks!