Test is supposed to verify that the corresponding source bundles are
there and this should be independent of whether it's coming from source
feature or not.
Test has been extended to actually check that there are source bundles
for every bundle in the feature as it was only checking that there are
target bundles for all source bundles but if a source bundle was missing
that could have easily be missed.
Side effect assertion about non-source bundle in source feature is being
dropped as source features are on their way out.
Fixes https://github.com/eclipse-pde/eclipse.pde/issues/1838 .