nandtest: seed random generator with time

nandtest: seed random generator with time

If a seed is not provided via --seed, we use the default rand() values,
which produces the same sequence of values every run. Since this is
undesirable, we should choose a random seed via the current time().

Note that this patch moves the srand() until after all the initial
options processing.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@intel.com>