Add timesteps per cycle option

Some VCD files will take more than one timestep for a single
clock-cycle. The previous commit to fix skipped clock cycles
did not take this into account. This commit adds options which
explicitly set the number of timesteps per clock cycle in each
of the two VCD files.

Signed-off-by: Palmer Dabbelt <palmer@dabbelt.com>
Add tests for new command-line arguments

Previous commits added command line arguments for raising signals up in
the module hierarchy and setting # of timesteps per cycle. This commit
adds tests for these features, along with a prerequisite change to the
testing harness. The testing harness now reads command-line arguments
from a temporary file.

Signed-off-by: Palmer Dabbelt <palmer@dabbelt.com>