> that actually changed the functionality under test - that's all an argument for doing exactly the opposite of what you recommend.
It was an old functionality. Someone wrote a super class that for the need of testing filesystem functionality created extremely large images. Not only there was no need to test with such large images, other developers eventually inherited more testcases from that setup code (because there were other utility methods), and now setUp code was needlessly creating images that no test used.
Generating a huge 4k image takes a significant time using Pillow.
It was an old functionality. Someone wrote a super class that for the need of testing filesystem functionality created extremely large images. Not only there was no need to test with such large images, other developers eventually inherited more testcases from that setup code (because there were other utility methods), and now setUp code was needlessly creating images that no test used.
Generating a huge 4k image takes a significant time using Pillow.