Add a unit test to check that maploaded simple/basic mobs are in an environment they can survive in. (#82180)

## About The Pull Request
I've recently noticed that the maploaded penguins from the snowdin away
mission were dying from unsafe atmos/temperature. This sparked the idea
of making a (focus only) unit test that would prevent this sort of
issues from happening.

This PR also implements the usage of the `atmos_requirements` and
`body_temp_sensitive` elements for simple animals too, cutting down the
copypaste.

## Why It's Good For The Game
More unit tests to make sure things are done correctly.

## Changelog

🆑
fix: Made sure that mapped critters (i.e. penguins on the snow cabin
away mission) can survive in the environment they're spawned in.
/🆑

code/modules/mob/living/basic/jungle/mega_arachnid/mega_arachnid.dm

@@ -16,7 +16,7 @@
 	pixel_x = -16
 	base_pixel_x = -16
 
-	habitable_atmos = list("min_oxy" = 0, "max_oxy" = 0, "min_plas" = 0, "max_plas" = 0, "min_co2" = 0, "max_co2" = 0, "min_n2" = 0, "max_n2" = 0)
+	habitable_atmos = null
 	faction = list(FACTION_JUNGLE)
 	obj_damage = 30
 	environment_smash = ENVIRONMENT_SMASH_WALLS

code/modules/mob/living/basic/jungle/venus_human_trap.dm

@@ -148,7 +148,7 @@
 	attack_sound = 'sound/creatures/venus_trap_hit.ogg'
 	unsuitable_heat_damage = 5 // heat damage is different from cold damage since coldmos is significantly more common than plasmafires
 	unsuitable_cold_damage = 2 // they now do take cold damage, but this should be sufficiently small that it does not cause major issues
-	habitable_atmos = list("min_oxy" = 0, "max_oxy" = 0, "min_plas" = 0, "max_plas" = 0, "min_co2" = 0, "max_co2" = 0, "min_n2" = 0, "max_n2" = 0)
+	habitable_atmos = null
 	unsuitable_atmos_damage = 0
 	/// copied over from the code from eyeballs (the mob) to make it easier for venus human traps to see in kudzu that doesn't have the transparency mutation
 	sight = SEE_SELF|SEE_MOBS|SEE_OBJS|SEE_TURFS