replace test case for slug character conversion

This scenario now tests if slug is left intact if explicitly specified

Author: mabezi

tests/unit/module_utils/test_data/netbox_utils/normalize_data.json

@@ -217,10 +217,10 @@
     },
     {
         "before": {
-            "slug": "Test to_slug"
+            "slug": "already--valid_slug"
         },
         "after": {
-            "slug": "test-to_slug"
+            "slug": "already--valid_slug"
         }
     },
     {
@@ -345,4 +345,4 @@
             "mac_address": "AA:BB:CC:DD:EE:FF"
         }
     }
-]
+]