Add test suite coverage

[197g]

We should use the test cases pack provided by libtiff, the newest version (3.8) can be obtained here: http://simplesystems.org/libtiff/images.html (~5.5 MB unpacked).

[197g]

Note that at least one image — caspian.tif — contains a private tag (42212/0xa480, see https://www.awaresystems.be/imaging/tiff/tifftags/private.html). See #47 for a discussion on handling these.