Add tests for optimal tile dimensions

These tests aren't comprehensive, but will catch problems like #10259.
Flex: fix NPEs in optimal tile size retrieval

Fixes ticket #10259.
Fix a few more tile size bugs
Imaris HDF: fix optimal tile height

Fixes ticket #10272.
Relax tile tests to allow sizes greater than SizeX or SizeY

Many TIFF files will have a tile size greater than the image width or
height, often so that the tile size is a multiple of 8 or power of 2.
BD Pathway: fix optimal tile height calculation

This prevents the optimal height from being 0.
Merge pull request #375 from melissalinkert/10259

Fix tile size retrieval for Flex