Fix Cadence Incisive compile warning

The SystemVerilog LRM (IEEE 1800-2012) clause 20.15.1 ($random
function) says: "The seed argument shall be an integral variable."

This fixes the following compile warning:

    rand_value = $random($urandom);
                       |
ncelab: *W,WRNOTL (/home/scottj/rocket-chip/vsrc/TestDriver.v,34|23): Argument to out parameter is not a legal lvalue.