Add more test cases for PNG

[fnands]

More test cases, and maybe a test framework need to be added so we can be sure that all supported PNG images should be readable.

Also, it might be nice to add some kind of framework to automatically run all tests.
Maybe: https://github.com/guidorice/mojo-pytest

[fnands]

Should probably just copy all of the cases from PIL here: https://github.com/python-pillow/Pillow/tree/main/Tests/images