Merge pull request rails#53204 from kabirpathak/update-testing-guide-to-use-assert-not

Update testing guide to use `assert_not` instead of `refute` [skip ci]

Author: fatkodima

guides/source/testing.md

@@ -2224,7 +2224,7 @@ class BillingJobTest < ActiveJob::TestCase
     assert_raises(InsufficientFundsError) do
       BillingJob.new(empty_account, product).perform
     end
-    refute account.reload.charged_for?(product)
+    assert_not account.reload.charged_for?(product)
   end
 end
 ```