Add libtiffpic test suite

Two tests are disabled, both related to tiles, since they do not follow
the fully released spec themselves but incidentally decode with libtiff
anyways. The two images (`cramps-tile.tif`, `quad-tile.tif`) are tiled
layout which libtiff detects by the presence of TileWidth/TileHeight
tags. We would detect that by chunks encoded via TileOffsets and
TileByteLengths tags. However the images encoded their chunk data into
the StripOffsets tag; additionally they contain a RowsPerStrip value
anyways.

Tests disabled as this is to be resolved in a separate issue (#338) with
a decision on treatment of this situation (strictness setting or copying
libtiff, also error late for mismatched lengths).

tests/COPYRIGHT

@@ -6,3 +6,7 @@ usda/*.tif
   url: https://registry.opendata.aws/naip/index.html
   credit: NAIP on AWS was accessed on 2026-01-28 from https://registry.opendata.aws/naip.
   license: Public Domain
+libtiffpic/
+  url: https://download.osgeo.org/libtiff/
+  credit: libtiff
+  license: CC0