Kenneth Solomon, the accused getaway driver in the 1982 Hancock Park slaying of Princeton University basketball player Lawrence Raphael, has pleaded guilty to first-degree murder--but he will be allowed to go free if he testifies truthfully in the pending trial of alleged triggerman Ozell (Junebug) Johnson. Deputy Dist. Atty. Stephen M. Barshop said he accepted the unusual plea bargain because Solomon, 29, is "a critical witness in the case" against Johnson. Solomon and Johnson, both of South-Central Los Angeles, were arrested in February, 1984.