Tests: spread the two longest sweeps over the days

swept_blocks_match_the_pixel_search skips the cases where the colour mode reaches extrude only
through a channel count and ink setting an earlier mode already covered. It and
damaged_layer_data_opens_flattened run a quarter of their cases per run, a different quarter each
UTC day (photocraft-testkit's Sweep). PHOTOCRAFT_FULL_SWEEP=1 runs every case, and
PHOTOCRAFT_SWEEP_PART=<n> replays the part a failing run printed.

Cargo.lock

@@ -4699,6 +4699,7 @@ dependencies = [
  "photocraft-color",
  "photocraft-geom",
  "photocraft-raster",
+ "photocraft-testkit",
  "rayon",
  "rustfft",
  "serde",
@@ -4921,6 +4922,7 @@ dependencies = [
  "photocraft-psd",
  "photocraft-raster",
  "photocraft-raw",
+ "photocraft-testkit",
  "photocraft-text",
  "photocraft-vector",
  "proptest",