Make can_inject deterministic; fix odysseus syringes sometimes not working [PR #12858, take two] (#14637)

* PR #12858 revival

* Move the can_inject unit tests to unit_tests/

* Remove the can_inject unit tests

code/modules/hydroponics/plant_genes.dm

@@ -396,7 +396,10 @@
 /datum/plant_gene/trait/stinging/on_throw_impact(obj/item/reagent_containers/food/snacks/grown/G, atom/target)
 	if(isliving(target) && G.reagents && G.reagents.total_volume)
 		var/mob/living/L = target
-		if(L.reagents && L.can_inject(null, FALSE))
+		// It would be nice to inject the body part the original thrower aimed at,
+		// but we don't have this kind of information here. So pick something at random.
+		var/target_zone = pick("chest", "chest", "chest", "l_leg", "r_leg", "l_arm", "r_arm", "head")
+		if(L.reagents && L.can_inject(null, FALSE, target_zone))
 			var/injecting_amount = max(1, G.seed.potency*0.2) // Minimum of 1, max of 20
 			var/fraction = min(injecting_amount/G.reagents.total_volume, 1)
 			G.reagents.reaction(L, REAGENT_INGEST, fraction)

code/modules/mob/living/carbon/human/human.dm

@@ -1021,34 +1021,34 @@
 	return
 
 /mob/living/carbon/human/can_inject(mob/user, error_msg, target_zone, penetrate_thick = FALSE)
-	. = 1
+	. = TRUE
 
 	if(!target_zone)
 		if(!user)
-			target_zone = pick("chest","chest","chest","left leg","right leg","left arm", "right arm", "head")
+			. = FALSE
+			CRASH("can_inject() called on a human mob with neither a user nor a targeting zone selected.")
 		else
 			target_zone = user.zone_selected
 
-
 	if(PIERCEIMMUNE in dna.species.species_traits)
-		. = 0
+		. = FALSE
 
 	var/obj/item/organ/external/affecting = get_organ(target_zone)
 	var/fail_msg
 	if(!affecting)
-		. = 0
+		. = FALSE
 		fail_msg = "[p_they(TRUE)] [p_are()] missing that limb."
 	else if(affecting.is_robotic())
-		. = 0
+		. = FALSE
 		fail_msg = "That limb is robotic."
 	else
 		switch(target_zone)
 			if("head")
 				if(head && head.flags & THICKMATERIAL && !penetrate_thick)
-					. = 0
+					. = FALSE
 			else
 				if(wear_suit && wear_suit.flags & THICKMATERIAL && !penetrate_thick)
-					. = 0
+					. = FALSE
 	if(!. && error_msg && user)
 		if(!fail_msg)
 			fail_msg = "There is no exposed flesh or thin material [target_zone == "head" ? "on [p_their()] head" : "on [p_their()] body"] to inject into."