Add tests

[hatkidchan]

TODO: add test target to Makefile, as well as test scripts (in Python, Bash or even in C itself?).

Test scripts should download sample images from this issue (TODO: add images) into obj/ folder.

Test scripts should run after running testbuild target or as last step of it. Test scripts should depend on all target.

help wanted label is set, because I need some sample images, if possible. SFW-only, public domain and/or royalty-free only. Self-made content also allowed. Images should be in either PNG or BMP (i.e. lossless compression only) to reduce ambiguity caused by (de-)compression.

[hatkidchan]

Test cases should include, but not limit to:

• Testing for valid JSON in -F json
• Testing for correct width/height in -F json
• Testing for correct pixel values
• Testing for hanging color after -F ansi
• Testing for valid table structure in -F html
• Testing for valid palette conversion

TODO: more test cases