Apply some changelog format tests to changelog directory

This commit applies some changelog format tests to changelog directory
to prevent the following error when running `rake changelog:merge`.

```console
% bundle exec rake changelog:merge
rake aborted!
NoMethodError: undefined method `[]' for nil:NilClass

  entry.match(/\. \((?<contributors>.+)\)\n/)[:contributors].split(',')
                                             ^^^^^^^^^^^^^^^
/Users/koic/src/github.com/rubocop/rubocop-minitest/tasks/changelog.rb:130:in `block in contributors'
/Users/koic/src/github.com/rubocop/rubocop-minitest/tasks/changelog.rb:129:in `each'
/Users/koic/src/github.com/rubocop/rubocop-minitest/tasks/changelog.rb:129:in `flat_map'
/Users/koic/src/github.com/rubocop/rubocop-minitest/tasks/changelog.rb:129:in `contributors'
/Users/koic/src/github.com/rubocop/rubocop-minitest/tasks/changelog.rb:123:in `new_contributor_lines'
/Users/koic/src/github.com/rubocop/rubocop-minitest/tasks/changelog.rb:105:in `merge_content'
/Users/koic/src/github.com/rubocop/rubocop-minitest/tasks/changelog.rb:94:in `merge!'
tasks/changelog.rake:21:in `block (2 levels) in <top (required)>'
/Users/koic/.rbenv/versions/3.1.2/bin/bundle:25:in `load'
/Users/koic/.rbenv/versions/3.1.2/bin/bundle:25:in `<main>'
Tasks: TOP => changelog:merge
(See full trace by running task with --trace)
```

This error occurs when there is no period in the changelog format. It is because the tests
applied to CHANGELOG.md were not applied to the changelog directory.

test/project_test.rb

@@ -4,6 +4,9 @@
 
 class ProjectTest < Minitest::Test
   def setup
+    @issues = []
+    @bodies = []
+
     load_changelog
     load_feature_entries
   end
@@ -133,18 +136,24 @@ def load_changelog
     @lines = @changelog.each_line
     @entries = @lines.grep(/^\*/).map(&:chomp)
 
-    @issues = @entries.map do |entry|
-      entry.match(/\[(?<number>[#\d]+)\]\((?<url>[^)]+)\)/)
-    end.compact
-
-    @bodies = @entries.map do |entry|
-      entry.gsub(/`[^`]+`/, '``').sub(/^\*\s*(?:\[.+?\):\s*)?/, '').sub(/\s*\([^)]+\)$/, '')
-    end
+    prepare_changelog_entries(@entries)
   end
 
   def load_feature_entries
     changelog_dir = File.join(File.dirname(__FILE__), '..', 'changelog')
 
     @feature_entries = Dir["#{changelog_dir}/*.md"]
+
+    prepare_changelog_entries(@feature_entries)
+  end
+
+  def prepare_changelog_entries(entries)
+    @issues += entries.map do |entry|
+      entry.match(/\[(?<number>[#\d]+)\]\((?<url>[^)]+)\)/)
+    end.compact
+
+    @bodies += entries.map do |entry|
+      entry.gsub(/`[^`]+`/, '``').sub(/^\*\s*(?:\[.+?\):\s*)?/, '').sub(/\s*\([^)]+\)$/, '')
+    end
   end
 end