libre: Add java-assertj
    
    This isn't the latest java-assertj but for now we'll use the 3.8.0
    version as it has less dependencies.
    
    It also worked fine in Guix so it was less likely to have problematic
    dependencies.
    
    Signed-off-by: Denis 'GNUtoo' Carikli <GNUtoo@cyberdimension.org>