Integration: move eyedropper_ring tests into tests/it (#7021)

Main now builds each crate's integration tests as one binary (crates/ui-egui/tests/it). The PR's
tests/eyedropper_ring.rs moves to tests/it/eyedropper_ring.rs behind `mod eyedropper_ring;`,
and both wgpu tests take photocraft_testkit::gpu_lock() like their neighbours.
ring_reads_over_a_flat_area_of_its_own_colour still fails with main's canvas.rs and passes with the PR's.

Co-authored-by: mroxso <24775431+mroxso@users.noreply.github.com>
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

crates/ui-egui/tests/it/eyedropper_ring.rs

@@ -105,6 +105,7 @@ fn outer_edge_reads(img: &Shot, c: Pos2) -> bool {
 
 #[test]
 fn ring_reads_over_a_flat_area_of_its_own_colour() {
+    let _gpu = photocraft_testkit::gpu_lock();
     let Some(mut h) = harness() else { return };
     h.run_steps(4);
     let c = setup(&mut h);
@@ -126,6 +127,7 @@ fn ring_reads_over_a_flat_area_of_its_own_colour() {
 
 #[test]
 fn show_sampling_ring_off_draws_nothing() {
+    let _gpu = photocraft_testkit::gpu_lock();
     let Some(mut h) = harness() else { return };
     h.run_steps(4);
     let c = setup(&mut h);

crates/ui-egui/tests/it/main.rs

@@ -17,6 +17,7 @@ mod custom_print_paper;
 mod display_shadows;
 mod drag_preview_canvas;
 mod export_as_scaling;
+mod eyedropper_ring;
 mod full_res_preview;
 mod gpu_device_loss;
 mod layout_perf;