The test lighting for a shader should be representative of the type of lighting that will be used in the production. For example, if you have HDRI environments in a lot of shots, you should have one when testing the shaders. If you use a lot of really bright rim light, you should test the shader with it. Same goes for whether or not it’ll have raytraced self-reflection, shadow maps or raytraced shadows, etc. If you know what a typical background plate will be where the shader will appear, then run the tests with that as your background.
Shader/texture tests are usually done on the actual model, after the model is approved. If the model isn’t built yet at least try to use a stand-in that’s as close as possible.
Talking about 50% gray and so on could be done in different contexts, I won’t post any guesses about what the person meant who said that.