Unit tests radio, saymode, and language prefix keys (#71328)

## About The Pull Request
This PR unit tests radio/saymode prefixes together, and language
prefixes on their own, for any possible overlaps and duplicates

Moved mafia saymode key to :1 and holopad key to :2, we're running out
of radio keys and i'm too afraid to move to 2-letter

## Why It's Good For The Game
1. Unit tests are good
2. https://github.com/tgstation/tgstation/pull/71326 this is bad

🆑 
spellcheck: Mafia changeling say prefix is now :1, and holopad say is
now :2
/🆑

Co-authored-by: Zephyr <12817816+ZephyrTFA@users.noreply.github.com>

code/modules/unit_tests/_unit_tests.dm

@@ -120,6 +120,7 @@
 #include "inhands.dm"
 #include "json_savefile_importing.dm"
 #include "keybinding_init.dm"
+#include "key_prefixes.dm"
 #include "knockoff_component.dm"
 #include "limbsanity.dm"
 #include "load_map_security.dm"

code/modules/unit_tests/key_prefixes.dm

@@ -0,0 +1,26 @@
+/// A unit test to ensure radio and language prefix keys don't overlap
+/datum/unit_test/key_prefixes
+
+/datum/unit_test/key_prefixes/Run()
+	// Languages first
+	// Assoc list of key:lang type
+	var/list/language_keys = list()
+	for(var/datum/language/language_type as anything in subtypesof(/datum/language))
+		if(initial(language_type.key) in language_keys)
+			TEST_FAIL("[language_type] language has the same prefix key ([initial(language_type.key)]) as [language_keys[initial(language_type.key)]]")
+		language_keys[initial(language_type.key)] = language_type
+
+	// Now radioes
+
+	// Assoc list of key:name (or type in the cases of saymodes)
+	var/list/radio_keys = list()
+
+	for(var/radio_key in GLOB.department_radio_keys)
+		if(radio_key in radio_keys)
+			TEST_FAIL("[GLOB.department_radio_keys[radio_key]] radio channel has the same prefix key ([radio_key]) as [GLOB.department_radio_keys[radio_keys.Find(radio_key)]]")
+		radio_keys[radio_key] = GLOB.department_radio_keys[radio_key]
+
+	for(var/datum/saymode/say_type as anything in subtypesof(/datum/saymode))
+		if(initial(say_type.key) in radio_keys)
+			TEST_FAIL("[say_type] saymode has the same prefix key ([initial(say_type.key)]) as [radio_keys[initial(say_type.key)]]")
+		radio_keys[initial(say_type.key)] = say_type