fix(testing): ensure mod validity in get_test_registrator

Check if the provided mod name is valid by iterating through the registered mod names before allowing test registration.

base.lua

@@ -44,6 +44,15 @@ local register_test = function(mod, name, func, opts)
 end
 
 test_harness.get_test_registrator = function(mod)
+    local modnames = minetest.get_modnames()
+    local is_mod = false
+    for i=1,#modnames do
+        if modnames[i]==mod then
+            is_mod=true
+            break
+        end
+    end
+    if not is_mod then error("get_test_registrator given mod "..mod.." is not a mod.") end
     return function(name, func, opts)
         register_test(mod, name, func, opts)
     end