add test for topic_compare

add test for `topic_compare.py`
* subscribe the same topic and they should say the same bps
* subscribe one topic and another topic with dropping 1 out of 2 messages (conducted by `topic_tools/drop`). compare them and the one should be the half of the other.

this is a test code for the following PR:
https://github.com/jsk-ros-pkg/jsk_common/pull/305