Add Avoid Double Negation With Minitest Refute as a Ruby TIL

ruby/avoid-double-negation-with-minitest-refute.md

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+# Avoid Double Negation With Minitest Refute
+
+As I'm writing some tests for a recent feature, I end up with various test
+cases that make a variety of assertions.
+
+```ruby
+assert_equal resp_body["id"], expected_id
+
+# ...
+
+assert_not resp_body["items"].empty?
+```
+
+The first assertion reads pretty naturally. The second one requires some extra
+mental parsing because of the `_not` and then the "negative" check of
+`#empty?`.
+
+Another way to express this that might read more naturally is with
+[`#refute`](https://ruby-doc.org/stdlib-3.0.2/libdoc/minitest/rdoc/Minitest/Assertions.html#method-i-refute).
+
+```ruby
+refute resp_body["items"].empty?
+```
+
+This says that we _refute_ that items is empty, so the assertion should fail if
+empty.
+
+Ruby is flexible in other ways. We may also prefer to write it as:
+
+```ruby
+assert resp_body["items"].present?
+```
+
+Or we could even take advantage of a more specific variant of refute with
+[`#refute_empty`](https://ruby-doc.org/stdlib-3.0.2/libdoc/minitest/rdoc/Minitest/Assertions.html#method-i-refute_empty):
+
+```ruby
+refute_empty resp_body["items"]
+```