Assign charges of velo tracks randomly

Currently, the charge of a VELO track in the conversion of PrVeloTrack to v1::Track is defined through its ChannelID:

    const int firstRow = outTr->lhcbIDs()[0].channelID();
    const int charge   = ( firstRow % 2 == 0 ? -1 : 1 );

However, this could give us non-deterministic behaviour in tests where we only swap the magnetic field, as pointed out by Marco: !3680 (comment 7403853) In addition, it's probably best to not have something random (charge) assigned by something physical (channelID).

Related: #524