When test discovery is invoked on a package on the filesystem which is also installed in site-packages then importing the tests will look in the installed version. This causes discovery to run the wrong tests or fail.

We can detect this (compare the __file__ against the expected location) and also insert the top level path as the first location in sys.path instead of appending it to the end.

If we detect a failed import then we should error out with an appropriate message.

(Nose gets round this by patching __import__ to import from a specific location which seems potentially fragile.)