Add minitest-proveit to development

This commit adds minitest-proveit to development and resolve the following errors:

```console
% bundle exec rake
(snip)

1) Failure:
ProjectTest#test_feature_entry_has_a_link_to_the_contributors_at_the_end
[/Users/koic/.rbenv/versions/3.2.0/lib/ruby/gems/3.2.0/gems/minitest-5.17.0/lib/minitest/test.rb:107]:
Absence of failure is not success. Prove it!

2) Failure:
ProjectTest#test_has_a_single_line
[/Users/koic/.rbenv/versions/3.2.0/lib/ruby/gems/3.2.0/gems/minitest-5.17.0/lib/minitest/test.rb:107]:
Absence of failure is not success. Prove it!

3) Failure:
ProjectTest#test_feature_entry_has_a_link_to_the_issue_or_pull_request_address_at_the_beginning
[/Users/koic/.rbenv/versions/3.2.0/lib/ruby/gems/3.2.0/gems/minitest-5.17.0/lib/minitest/test.rb:107]:
Absence of failure is not success. Prove it!

4) Failure:
ProjectTest#test_feature_entry_starts_with_new_fix_or_change
[/Users/koic/.rbenv/versions/3.2.0/lib/ruby/gems/3.2.0/gems/minitest-5.17.0/lib/minitest/test.rb:107]:
Absence of failure is not success. Prove it!

2220 runs, 6398 assertions, 4 failures, 0 errors, 0 skips
rake aborted!
```

Gemfile

@@ -7,6 +7,7 @@ git_source(:github) { |repo| "https://github.com/#{repo}.git" }
 gemspec
 
 gem 'bump', require: false
+gem 'minitest-proveit'
 gem 'rake'
 gem 'rubocop', github: 'rubocop/rubocop'
 gem 'rubocop-performance', '~> 1.15.0'

test/project_test.rb

@@ -91,6 +91,8 @@ def test_body_ends_with_a_punctuation
   end
 
   def test_feature_entry_has_a_link_to_the_issue_or_pull_request_address_at_the_beginning
+    skip if @feature_entries.empty?
+
     repo = 'rubocop/rubocop-minitest'
     address_pattern = %r{\A\* \[#\d+\]\(https://github\.com/#{repo}/(issues|pull)/\d+\):}
 
@@ -100,18 +102,24 @@ def test_feature_entry_has_a_link_to_the_issue_or_pull_request_address_at_the_be
   end
 
   def test_feature_entry_has_a_link_to_the_contributors_at_the_end
+    skip if @feature_entries.empty?
+
     @feature_entries.each do |path|
       assert_match(/\(\[@\S+\]\[\](?:, \[@\S+\]\[\])*\)$/, File.read(path))
     end
   end
 
   def test_feature_entry_starts_with_new_fix_or_change
+    skip if @feature_entries.empty?
+
     @feature_entries.each do |path|
       assert_match(/\A(new|fix|change)_.+/, File.basename(path))
     end
   end
 
   def test_has_a_single_line
+    skip if @feature_entries.empty?
+
     @feature_entries.each do |path|
       assert_equal(1, File.foreach(path).count)
     end

test/test_helper.rb

@@ -4,3 +4,8 @@
 
 # Require supporting files exposed for testing.
 require 'rubocop/minitest/support'
+require 'minitest/proveit'
+
+Minitest::Test.class_eval do
+  prove_it!
+end