[unnecessary-dict-index-lookup] Add a test case in the functional test

Following review, see https://github.com/PyCQA/pylint/pull/4640/files/10bdef17f0b079addd9e3af90f59a0e1952cfc90#r661698900

Author: Pierre-Sassoulas

tests/functional/u/unnecessary/unnecessary_dict_index_lookup.py

@@ -31,9 +31,12 @@
 class Foo:
     c_dict = {}
 
+
 for k, v in Foo.c_dict.items():
     print(b_dict[k])  # Should not emit warning, accessing other dictionary
     print(Foo.c_dict[k])  # [unnecessary-dict-index-lookup]
+    unnecessary = 0 # pylint: disable=invalid-name
+    unnecessary += Foo.c_dict[k]  # [unnecessary-dict-index-lookup]
     Foo.c_dict[k] += v  # key access necessary
 
 # Tests on comprehensions

tests/functional/u/unnecessary/unnecessary_dict_index_lookup.txt

@@ -7,16 +7,17 @@ unnecessary-dict-index-lookup:24:33::Unnecessary dictionary index lookup, use 'v
 unnecessary-dict-index-lookup:26:1::Unnecessary dictionary index lookup, use 'v' instead:HIGH
 unnecessary-dict-index-lookup:27:1::Unnecessary dictionary index lookup, use 'v' instead:HIGH
 unnecessary-dict-index-lookup:27:41::Unnecessary dictionary index lookup, use 'v' instead:HIGH
-unnecessary-dict-index-lookup:36:10::Unnecessary dictionary index lookup, use 'v' instead:HIGH
-unnecessary-dict-index-lookup:40:40::Unnecessary dictionary index lookup, use 'v' instead:HIGH
-unnecessary-dict-index-lookup:42:1::Unnecessary dictionary index lookup, use 'v' instead:HIGH
-unnecessary-dict-index-lookup:43:1::Unnecessary dictionary index lookup, use 'v' instead:HIGH
-unnecessary-dict-index-lookup:43:52::Unnecessary dictionary index lookup, use 'v' instead:HIGH
-unnecessary-dict-index-lookup:45:37::Unnecessary dictionary index lookup, use 'v' instead:HIGH
-unnecessary-dict-index-lookup:47:1::Unnecessary dictionary index lookup, use 'v' instead:HIGH
-unnecessary-dict-index-lookup:48:1::Unnecessary dictionary index lookup, use 'v' instead:HIGH
-unnecessary-dict-index-lookup:48:49::Unnecessary dictionary index lookup, use 'v' instead:HIGH
-unnecessary-dict-index-lookup:54:10::Unnecessary dictionary index lookup, use 'item[1]' instead:HIGH
-unnecessary-dict-index-lookup:57:1::Unnecessary dictionary index lookup, use 'item[1]' instead:HIGH
-unnecessary-dict-index-lookup:62:10::Unnecessary dictionary index lookup, use 'item[1]' instead:HIGH
-unnecessary-dict-index-lookup:79:14::Unnecessary dictionary index lookup, use '_' instead:HIGH
+unnecessary-dict-index-lookup:37:10::Unnecessary dictionary index lookup, use 'v' instead:HIGH
+unnecessary-dict-index-lookup:39:19::Unnecessary dictionary index lookup, use 'v' instead:HIGH
+unnecessary-dict-index-lookup:43:40::Unnecessary dictionary index lookup, use 'v' instead:HIGH
+unnecessary-dict-index-lookup:45:1::Unnecessary dictionary index lookup, use 'v' instead:HIGH
+unnecessary-dict-index-lookup:46:1::Unnecessary dictionary index lookup, use 'v' instead:HIGH
+unnecessary-dict-index-lookup:46:52::Unnecessary dictionary index lookup, use 'v' instead:HIGH
+unnecessary-dict-index-lookup:48:37::Unnecessary dictionary index lookup, use 'v' instead:HIGH
+unnecessary-dict-index-lookup:50:1::Unnecessary dictionary index lookup, use 'v' instead:HIGH
+unnecessary-dict-index-lookup:51:1::Unnecessary dictionary index lookup, use 'v' instead:HIGH
+unnecessary-dict-index-lookup:51:49::Unnecessary dictionary index lookup, use 'v' instead:HIGH
+unnecessary-dict-index-lookup:57:10::Unnecessary dictionary index lookup, use 'item[1]' instead:HIGH
+unnecessary-dict-index-lookup:60:1::Unnecessary dictionary index lookup, use 'item[1]' instead:HIGH
+unnecessary-dict-index-lookup:65:10::Unnecessary dictionary index lookup, use 'item[1]' instead:HIGH
+unnecessary-dict-index-lookup:82:14::Unnecessary dictionary index lookup, use '_' instead:HIGH