Use MiniTest::Unit::TestCase instead of Minitest::Test

Since Rails 4.0.x is depending on Minitest 4.x, the constant won't be
available so the gist won't run.

[ci skip]
Merge pull request #12166 from JuanitoFatas/doc-patch-1

[ci skip] Change link name of Rails i18n wiki.
Merge pull request #12157 from robin850/patch-9

Use MiniTest::Unit::TestCase instead of Minitest::Test  [ci skip]
[ci skip] Add missing setting locale code snippet.

This is for 3 Internationalizing your Application of i18n.md.

Missing the code as metentioned in 2.3, setting locale.

Without this, the example to add config/locales/pirate.yml will not work.
Fix links in section 5.3 of i18n.md

Now all links point to the correct line number.