[ci skip] MiniTest -> Minitest

MiniTest was renamed to Minitest.
Already renamed on rails#13366
But slipped into on https://github.com/rails/rails/pull/18413/files#diff-6bb90a693835b0e92910b796c8b0ef59R27

Author: mtsmfm

railties/test/code_statistics_calculator_test.rb

@@ -24,7 +24,7 @@ def foo
     end
   end
 
-  test "count number of methods in MiniTest file" do
+  test "count number of methods in Minitest file" do
     code = <<-RUBY
       class FooTest < ActionController::TestCase
         test 'expectation' do