test/rubocops/formula_desc: Ensure that desc allows ending with etc.

- This was funny (and embarrassing), I was looking at my first contributions to
  Homebrew since someone asked me how long I'd been involved.
- Turns out that in 2017 I improved the formula description RuboCop rule to
  allow descriptions to end with `etc.` in
  https://github.com/Homebrew/brew/pull/3411. Apparently I didn't have time
  back then to write a test for it. So now I will remedy this 5 years later.

Library/Homebrew/test/rubocops/formula_desc_spec.rb

@@ -128,6 +128,15 @@ describe RuboCop::Cop::FormulaAudit::Desc do
       RUBY
     end
 
+    it "does not report an offense when the description ends with 'etc.'" do
+      expect_no_offenses(<<~RUBY, "/homebrew-core/Formula/foo.rb")
+        class Foo < Formula
+          url 'https://brew.sh/foo-1.0.tgz'
+          desc 'Description of a thing and some more things and some more etc.'
+        end
+      RUBY
+    end
+
     it "reports and corrects all rules for description text" do
       expect_offense(<<~RUBY, "/homebrew-core/Formula/foo.rb")
         class Foo < Formula