Mark Minitest/RefuteFalse as unsafe

Follow up #209 (comment)

This PR marks `Minitest/RefuteFalse` as unsafe.

changelog/change_mark_minitest_refute_false_as_unsafe.md

@@ -0,0 +1 @@
+* [#233](https://github.com/rubocop/rubocop-minitest/issues/233): Mark `Minitest/RefuteFalse` as unsafe. ([@koic][])

config/default.yml

@@ -191,7 +191,9 @@ Minitest/RefuteFalse:
   Description: 'Check if your test uses `refute(actual)` instead of `assert_equal(false, actual)`.'
   StyleGuide: 'https://minitest.rubystyle.guide#refute-false'
   Enabled: true
+  Safe: false
   VersionAdded: '0.3'
+  VersionChanged: '<<next>>'
 
 Minitest/RefuteInDelta:
   Description: 'This cop enforces the test to use `refute_in_delta` instead of using `refute_equal` to compare floats.'

lib/rubocop/cop/minitest/refute_false.rb

@@ -5,6 +5,16 @@ module Cop
     module Minitest
       # Enforces the use of `refute(object)` over `assert_equal(false, object)`.
       #
+      # @safety
+      #   This cop is unsafe because it cannot detect failure when second argument is `nil`.
+      #   False positives cannot be prevented when this is a variable or method return value.
+      #
+      #   [source,ruby]
+      #   ----
+      #   assert_equal(false, nil) # failure
+      #   refute(nil)              # success
+      #   ----
+      #
       # @example
       #   # bad
       #   assert_equal(false, actual)