Mark Minitest/AssertTruthy as unsafe

This PR marks `Minitest/AssertTruthy` as unsafe.
As with rubocop#233, this is a false positive issue than autocorrection compatibility.

changelog/change_mark_minitest_assert_truthy_as_unsafe.md

@@ -0,0 +1 @@
+* [#234](https://github.com/rubocop/rubocop-minitest/pull/234): Mark `Minitest/AssertTruthy` as unsafe. ([@koic][])

config/default.yml

@@ -111,9 +111,9 @@ Minitest/AssertTruthy:
   Description: 'This cop enforces the test to use `assert(actual)` instead of using `assert_equal(true, actual)`.'
   StyleGuide: 'https://minitest.rubystyle.guide#assert-truthy'
   Enabled: true
-  SafeAutoCorrect: false
+  Safe: false
   VersionAdded: '0.2'
-  VersionChanged: '0.26'
+  VersionChanged: '<<next>>'
 
 Minitest/AssertWithExpectedArgument:
   Description: 'This cop tries to detect when a user accidentally used `assert` when they meant to use `assert_equal`.'

lib/rubocop/cop/minitest/assert_truthy.rb

@@ -6,7 +6,14 @@ module Minitest
       # Enforces the test to use `assert(actual)` instead of using `assert_equal(true, actual)`.
       #
       # @safety
-      #   This cop's autocorrection is unsafe because true might be expected instead of truthy.
+      #   This cop is unsafe because true might be expected instead of truthy.
+      #   False positives cannot be prevented when this is a variable or method return value.
+      #
+      #   [source,ruby]
+      #   ----
+      #   assert_equal(true, 'truthy') # failure
+      #   assert('truthy')             # success
+      #   ----
       #
       # @example
       #   # bad