Currently, if you define your own manifest in your test project (which you shouldn't, see faq) this is being merged with the on-the-fly one.
While most package definitions might merge up well, this leads to unexpected behaviour (test bundle itself can register services through its own activator).
A fix should detect such cases (manifest found) and stop running the tests imediately and show a message that explains the case.
test will follow later.
By now, if you try to add your own manifest you'll get a ConfigurationException explaining the details.