Disable Stable Diffusion XL test.

It's still broken when running on BuildKite, will figure out why tomorrow.

Disable it in the meantime.

PiperOrigin-RevId: 617398037

test/gpu/imagegen_test.go

@@ -25,6 +25,7 @@ import (
 
 // TestStableDiffusionXL generates an image with Stable Diffusion XL.
 func TestStableDiffusionXL(t *testing.T) {
+	t.Skip("TODO(b/306706609): Fix this test.")
 	ctx := context.Background()
 	sdxl := stablediffusion.NewDockerXL(t)
 	generateCtx, generateCancel := context.WithTimeout(ctx, 15*time.Minute)