Test for common

I've written some tests for common/, mainly with the intent of familiarizing myself with the codebase.

I'm told that e.g. the huffman decoder will be rewritten some time in the future - I plan on expanding tests/common/huffman.h sometime in the future, after studying carefully the literature.

I I have followed the coding and styling convention in good faith, but I may have missed out bits - I apologize.

Thank you