Add new Minitest/RedundantMessageArgument cop

This PR adds new `Minitest/RedundantMessageArgument` cop
that detects redundant message argument in assertion methods.
The message argument `nil` is redundant because it is the default value.

```ruby
# bad
assert_equal(expected, actual, nil)

# good
assert_equal(expected, actual)
assert_equal(expected, actual, 'message')
```

config/default.yml

@@ -198,6 +198,11 @@ Minitest/NonPublicTestMethod:
   Severity: warning
   VersionAdded: '0.27'
 
+Minitest/RedundantMessageArgument:
+  Description: 'Detects redundant message argument in assertion methods.'
+  Enabled: pending
+  VersionAdded: '<<next>>'
+
 Minitest/RefuteEmpty:
   Description: 'This cop enforces to use `refute_empty` instead of using `refute(object.empty?)`.'
   StyleGuide: 'https://minitest.rubystyle.guide#refute-empty'

lib/rubocop/cop/minitest/redundant_message_argument.rb

@@ -0,0 +1,88 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Minitest
+      # Detects redundant message argument in assertion methods.
+      # The message argument `nil` is redundant because it is the default value.
+      #
+      # @example
+      #
+      #   # bad
+      #   assert_equal(expected, actual, nil)
+      #
+      #   # good
+      #   assert_equal(expected, actual)
+      #   assert_equal(expected, actual, 'message')
+      #
+      class RedundantMessageArgument < Base
+        extend AutoCorrector
+
+        MSG = 'Remove the redundant message argument.'
+
+        RESTRICT_ON_SEND = %i[
+          assert assert_empty assert_equal assert_same assert_in_delta assert_in_epsilon assert_includes
+          assert_instance_of assert_kind_of assert_match assert_nil assert_operator assert_path_exists
+          assert_predicate assert_respond_to assert_same assert_throws
+          flunk
+          refute refute_empty refute_equal refute_in_delta refute_in_epsilon refute_includes
+          refute_instance_of refute_kind_of refute_match refute_nil refute_operator refute_path_exists
+          refute_predicate refute_respond_to refute_same
+        ].freeze
+
+        # @!method bad_method?(node)
+        def_node_matcher :redundant_message_argument, <<~PATTERN
+          {
+            (send nil? :assert _ $nil)
+            (send nil? :assert_empty _ $nil)
+            (send nil? :assert_equal _ _ $nil)
+            (send nil? :assert_in_delta _ _ _ $nil)
+            (send nil? :assert_in_epsilon _ _ _ $nil)
+            (send nil? :assert_includes _ _ $nil)
+            (send nil? :assert_instance_of _ _ $nil)
+            (send nil? :assert_kind_of _ _ $nil)
+            (send nil? :assert_match _ _ $nil)
+            (send nil? :assert_nil _ $nil)
+            (send nil? :assert_operator _ _ _ $nil)
+            (send nil? :assert_path_exists _ $nil)
+            (send nil? :assert_predicate _ _ $nil)
+            (send nil? :assert_respond_to _ _ $nil)
+            (send nil? :assert_same _ _ $nil)
+            (send nil? :assert_throws _ $nil)
+            (send nil? :flunk $nil)
+            (send nil? :refute _ $nil)
+            (send nil? :refute_empty _ $nil)
+            (send nil? :refute_equal _ _ $nil)
+            (send nil? :refute_in_delta _ _ _ $nil)
+            (send nil? :refute_in_epsilon _ _ _ $nil)
+            (send nil? :refute_includes _ _ $nil)
+            (send nil? :refute_instance_of _ _ $nil)
+            (send nil? :refute_kind_of _ _ $nil)
+            (send nil? :refute_match _ _ $nil)
+            (send nil? :refute_nil _ $nil)
+            (send nil? :refute_operator _ _ _ $nil)
+            (send nil? :refute_path_exists _ $nil)
+            (send nil? :refute_predicate _ _ $nil)
+            (send nil? :refute_respond_to _ _ $nil)
+            (send nil? :refute_same _ _ $nil)
+          }
+        PATTERN
+
+        def on_send(node)
+          return unless (redundant_message_argument = redundant_message_argument(node))
+
+          add_offense(redundant_message_argument) do |corrector|
+            if node.arguments.one?
+              range = redundant_message_argument
+            else
+              index = node.arguments.index(redundant_message_argument)
+              range = node.arguments[index - 1].source_range.end.join(redundant_message_argument.source_range.end)
+            end
+
+            corrector.remove(range)
+          end
+        end
+      end
+    end
+  end
+end

lib/rubocop/cop/minitest_cops.rb

@@ -30,6 +30,7 @@
 require_relative 'minitest/assert_truthy'
 require_relative 'minitest/duplicate_test_run'
 require_relative 'minitest/empty_line_before_assertion_methods'
+require_relative 'minitest/redundant_message_argument'
 require_relative 'minitest/return_in_test_method'
 require_relative 'minitest/test_file_name'
 require_relative 'minitest/global_expectations'