Optimize cardboard cutouts saving 1.5s+ on init times (#73404)

New regression in init times. Closes
https://github.com/tgstation/dev-cycles-initiative/issues/32. CC @Fikou

- Instead of creating a human and icon for *every* cardboard cutout when
initialized, only creates the one we're actually using. When you're
about to use a crayon, creates all of them.
- Instead of using getFlatIcon, uses appearances directly.

code/modules/unit_tests/_unit_tests.dm

@@ -91,6 +91,7 @@
 #include "breath.dm"
 #include "cable_powernets.dm"
 #include "card_mismatch.dm"
+#include "cardboard_cutouts.dm"
 #include "chain_pull_through_space.dm"
 #include "chat_filter.dm"
 #include "circuit_component_category.dm"

code/modules/unit_tests/cardboard_cutouts.dm

@@ -0,0 +1,27 @@
+/// Validates that cardboard cutouts have the proper icons
+/datum/unit_test/cardboard_cutouts
+
+/datum/unit_test/cardboard_cutouts/Run()
+	var/obj/item/cardboard_cutout/normal_cutout = new
+	test_screenshot("normal_cutout", getFlatIcon(normal_cutout))
+
+	var/obj/item/cardboard_cutout/nuclear_operative/nukie_cutout = new
+	test_screenshot("nukie_cutout", getFlatIcon(nukie_cutout))
+
+	nukie_cutout.push_over()
+	test_screenshot("nukie_cutout_pushed", getFlatIcon(nukie_cutout))
+
+#if DM_VERSION >= 515
+	// This is the only reason we're testing xenomorphs.
+	// Making a custom subtype with direct_icon is hacky.
+	ASSERT(!isnull(/datum/cardboard_cutout/xenomorph_maid::direct_icon))
+#endif
+
+	var/obj/item/cardboard_cutout/xenomorph/xenomorph_cutout = new
+	test_screenshot("xenomorph_cutout", getFlatIcon(xenomorph_cutout))
+
+/obj/item/cardboard_cutout/nuclear_operative
+	starting_cutout = "Nuclear Operative"
+
+/obj/item/cardboard_cutout/xenomorph
+	starting_cutout = "Xenomorph"

code/modules/unit_tests/screenshot_dynamic_human_icons.dm

@@ -3,5 +3,5 @@
 
 /datum/unit_test/screenshot_dynamic_human_icons/Run()
 	// Complicated MODsuit setup
-	var/icon/icon = get_dynamic_human_icon(/datum/outfit/syndicatecommandocorpse)
-	test_screenshot("syndicate_commando", icon)
+	var/appearance = get_dynamic_human_appearance(/datum/outfit/syndicatecommandocorpse)
+	test_screenshot("syndicate_commando", get_flat_icon_for_all_directions(appearance))