Support validating test classes in Maven Mojo

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The current MavenMojo class can only validate classes from the build/classes 
dir, not the test dir. It might be possible to do this with the same Mojo, but 
the problem here is the DependencyResolutionScope, which cannot be changed at 
runtime.
I think we must add a second Mojo (ideally all Mojos using the same default 
base class, only providing access to defaults and classpath)

Original issue reported on code.google.com by uwe.h.schindler on 12 Feb 2013 at 8:25