xAODRootAccess: Skip InheritsFrom tests that can't succeed.
In THolder, we were testing whether a type derives from SG::AuxVectorBase or SG::AuxElement using TClass::InheritsFrom(). Change to test the STLtype first --- an AuxElement will be kNoSTL and a DataVector will be kSTLlist.
This saves some time, and possibly autoparsing, which in turn avoids a cling assertion failure seen while running the unit tests in the debug build.