Refactor and add tests for plugins.py

Improvement to the tests currently being run on ``plugins.py``

related #644 
Ah, these numbers look far more magical outside of the text editor..
These numbers are the number of plugins in each category on the linux2 platform.
I can look into yapsy for how it detects the plugins. That would mean that a function is created which can also count how many plugins are available in the freeseer plugin directories (I am not sure how much work that is), which could be error prone as well.

I don't think it is possible to make the test any more flexible by asserting that specific plugins are in there as well. Because a developer may also remove or add a plugin.

It could be possible to make a set of fake plugins, then test every thing with respect to those?