Add test fixture.

src/image/png/PngWriter.cpp

@@ -200,7 +200,6 @@ void PngWriter::write(const std::unique_ptr<Image<unsigned char> >& image)
         {
             auto huffman_encoder = std::make_unique<HuffmanEncoder>();
             huffman_encoder->setUseFixedCode(true);
-            huffman_encoder->initializeTrees();
             lz77_encoder.setPrefixCodeGenerator(std::move(huffman_encoder));
         }
         lz77_encoder.encode();