Simplify tests

[radarhere]

Suggestions for python-pillow#7209

• Move sample bytes inside test. I expect you had it outside the test so that it could be reused, but I don't think we value speed quite that highly in the test suite
• Simplified the code by using assert_image_equal() instead of asserting the results of getpixel()
• The number of bands is no longer used, so I've removed them.
• Added type hints for the return values of the tests
• Only specify the image size once in a test. This is fairly minor, but it feels neater