plugin should understand sourceModifications

As it stands, there's no way of changing which files get compiled from the <unitTestSourceDirectory/>

For consistency, it would make sense if this were controllable via the sourceModification tag. I will attach a patch.

However, it might be nicer to have a <unitTestSourceModifications/> tag added to the pom, which would need more significant work (and a upwards revision of how long this would take to solve...).