Test suite buildstream imports may be ambiguous

When running import buildstream in our tests, python decides which buildstream to import.

If this is done on a machine with a separate buildstream installation in another path (say, a dev machine that has buildstream installed in /usr/lib/something), we might accidentally test the wrong version of buildstream - this would be quite hard to work around for the tester, too.

Ways to mitigate this would be to either use tox, or to use a custom import wrapper in testing.