Some time ago I decided to locate, rename (according to the TOSEC naming convention) and collect all those amazing pieces of software that many people have been writing over the last decades to test their emulators. Some of them have been really hard to track down, as they were usually posted on forums or pages that have long gone offine.
I always try to get the original files, preserving the original modification date where possible (for historical reasons), and taking note of where I get them from to provide some context.
I've put them all on two wikis on GitHub:
https://github.com/redcode/Z80/wiki/Tests
https://github.com/redcode/ZXSpectrum/wiki/Tests
In the future, I will move this material to https://zxspectrum.dev (it's not online yet).
If you know of other tests, please post a comment here or open a discussion on GitHub.