Enable the clippy::std_instead_of_core lint (#8394)

Prefer `core::` over `std::` where either work

* Part of https://github.com/emilk/egui/issues/5735

crates/egui_kittest/src/renderer.rs

@@ -1,5 +1,5 @@
+use core::mem;
 use egui::TexturesDelta;
-use std::mem;
 
 pub trait TestRenderer {
     /// We use this to pass the glow / wgpu render state to [`eframe::Frame`].