Bring the location of the ipsilon executable in coherence
    
    The tests expect the executable to be named `ipsilon` too, as in the
    source code directory.
    
    This reverts commit 83e73742887dc45c388dde39ec6695e4c9b6f756.
    
    Signed-off-by: Aurélien Bompard <aurelien@bompard.org>