Collect a test suite of vcd files

[yufengwng]

To better test our vcd-parser and the fliplot waveform viewer, it'll be great if we have more example vcd files to work with. We can look for sample vcd files around the web. Another thing to do is go over the verilog questions in CS 233 course, do those questions, and grab the vcd files.

[yufengwng]

Added pm.vcd which is a larger file that parser is able to correctly parse. We'll need more examples.