advanced test suite failing due to relative import error

[ThibHlln]

For some users, the use of relative imports for DummyC and DummyFortran components fails. The tests are structured as a package of its own, yet it includes some scripts to be run inside the package, which goes against the pattern for use of relative imports. See some explanations on SO: https://stackoverflow.com/a/16985066

It would be good to restructure the tests as a proper package of its own, and extract the run_all_tests.py and run_basic_tests.py from the package to become "users" of the package.