TEST: tests for common/huffman.h

A few tests for the Huffman decoder.

The encoding is the example from Wikipedia.
This could be improved by someone more knowledgeable by
generating one at runtime or using multiple encodings
which would each contain one edge case.
TEST: test cases for common/util.cpp
TEST: tests for common/rendermode.h

tests for parseRenderMode, renderMode2GUIO and getRenderModeCode.
TEST: reorganize test/common/util.h

Split a few test cases in two.
The resulting file is now better organized.
TEST: Thorough testing for common/hash-str.h

We test the various equal_to and hash functions therein.
TEST: removed fixed hashes in test/common/hash-str.h

The hash function does not necessarily have to conform to one specific algorithm as long as equals/differs is respected.
TEST: Whitespaces in test/common/