Handle ViewportCommand::Screenshot in kittest (#8256)

Makes it possible to use egui_mcp to drive apps headlessly via kittest

crates/egui_kittest/Cargo.toml

@@ -37,6 +37,7 @@ x11 = ["eframe?/x11"]
 egui.workspace = true
 eframe = { workspace = true, optional = true }
 kittest.workspace = true
+log.workspace = true
 serde.workspace = true
 toml = { workspace = true, features = ["parse", "serde"] }