Re-implement PHPUnit tests for class-oik-attachment-contents.php

[bobbingwide]

Originally developed for bobbingwide/oik-bob-bing-wide#40 and then lost due to stupidity as documented in bobbingwide/oik-bob-bing-wide#41 the tests for class Oik_attachment_contents should be rewritten and included in the oik-libs plugin, since this is supposed to be the master repository of shared library files.

Note: I still don't use composer or packages.

The tests should cover those that I developed when writing the documentation for oik-bbw/csv, oik-css/css and oik-css/geshi. They neededn't/shouldn't cover the actual logic of these blocks or their associated shortcodes.