libre: Add opentest4j
    
    Opentest4j is now a dependency of newer versions of junit.
    
    The upstream URL of opentest4j was verified with[1].
    
    [1]https://github.com/junit-team/junit5/issues/12
    
    Signed-off-by: Denis 'GNUtoo' Carikli <GNUtoo@cyberdimension.org>