Adds a unit test that makes sure subsystem flags make sense (#96022)

## About The Pull Request

There's a few of these that conflict, mostly relating to timing. This
conflicting won't actually break anything but it does muddy "what is
this doing" somewhat and it's good to be clear so here we go

## Why It's Good For The Game

Makes the MC very slightly harder to confuse yourself with

code/modules/admin/view_variables/debug_variables.dm

@@ -103,20 +103,14 @@
 		return "<a href='byond://?_src_=vars;[HrefToken()];[link_vars]'>/list ([list_value.len])</a>"
 
 	// if it's a number, is it a bitflag?
-	var/list/valid_bitflags
-	if(!isnum(name))
-		valid_bitflags = get_valid_bitflags(name)
+	var/list/matching_bitflags = get_matching_bitflags(name, value)
 
-	if(!length(valid_bitflags))
-		return "<span class='value'>[VV_HTML_ENCODE(value)]</span>"
+	if(!isnull(matching_bitflags))
+		if(length(matching_bitflags))
+			return "[VV_HTML_ENCODE(matching_bitflags.Join(", "))]"
+		return "NONE"
 
-	var/list/flags = list()
-	for (var/bit_name in valid_bitflags)
-		if (value & valid_bitflags[bit_name])
-			flags += bit_name
-	if(length(flags))
-		return "[VV_HTML_ENCODE(flags.Join(", "))]"
-	return "NONE"
+	return "<span class='value'>[VV_HTML_ENCODE(value)]</span>"
 
 /datum/proc/debug_variable_value(name, level, datum/owner, sanitize, display_flags)
 	if("[src]" != "[type]") // If we have a name var, let's use it.