redefine %_rpmfilename to match the default
    
    The %_rpmfilename macro dictates the output filename pattern for binary
    packages produced by rpmbuild.
    
    Rpmfluff is expecting to find the binary packages according to the
    default pattern shipped with rpm, but something else may have redefined
    this macro causing the binary packages to be written to a different
    location.
    
    For example, mock redefines %_rpmfilename to drop the arch subdirectory,
    which causes rpmfluff to fail:
    
        cp: cannot stat 'test-rpmbuild-b-0.1-1/RPMS/i386/b-0.1-1.i386.rpm': No such file or directory
    
    because the file is actually at RPMS/b-0.1-1.i386.rpm instead.