Be aware that the open-source GL tests in vk-gl-cts still contain quite a lot of bugs from porting from the old framework that was used in proprietary test suite releases.
Piglit isn't really a conformance test suite. It is a test suite, is usually extended when people add new features to Mesa, and collects regression tests over time. However, it actually started out in part as a way to modify glean tests that drivers couldn't pass because the hardware was lacking, for example, hardware didn't have enough precision in the blender... those were the days. The p in piglit stands for your choice of pragmatic or practical. I'm not sure if that's actually documented anywhere, but as the original author, I should know ;-)
I'm sure piglit has bugs, but it's also well known that certain closed source drivers have less than conformant GLSL compilers, for example - so the fact that drivers passing the Khronos conformance tests fail piglit tests in itself doesn't mean anything other than shit needs investigating, and occasionally needs spec clarifications.
There was a pixel-accurate test suite for OpenGL 1,0 from Kronos,[1] but it predates programmable shaders. It tests the geometry and texture processing. It's not free.