Currently if the user uses a JUnit Classpath container one still needs
to add import package/require bundle to the manifest to make these
usable inside the running OSGi framework. This is not only cumbersome
but also not desired if for example the project uses a testfolder inside
the plugin. Also other libraries might be affected in a similar way if
they come from other classpath containers.

This now detects if a JUnit Bundle Container is used and then compute an
auxiliary manifest that is attached to the testprobe with a fragment,
this is similar to how Tycho behaves already. With that it is now
possible to much more smoothly use such setups and will enabled advanced
use-cases in the future.