Add simple integration test to just run housekeeper

In https://gitlab.com/gitlab-org/gitlab/-/merge_requests/181613 you can
see it was easy to accidentally completely break housekeeper as there is
no integration test for it. That kind of problem (missing a `require`)
cannot be spotted by running RSpec because the order dependence of tests
might (and did) mask the issue.

Adding an integration test here is an easy way to reduce the risk of this
happening again.

gems/gitlab-housekeeper/.gitlab-ci.yml

@@ -2,3 +2,15 @@ include:
   - local: gems/gem.gitlab-ci.yml
     inputs:
       gem_name: "gitlab-housekeeper"
+
+gitlab-housekeeper-integration-test:
+  script:
+    - mkdir -p keeps
+    - cp spec/fixtures/pretty_useless_keep.rb keeps/
+    - git init --initial-branch=master
+    - 'git config --global user.email "test@example.com"'
+    - 'git config --global user.name "Test User"'
+    - 'git commit --allow-empty -m "Initial commit"'
+    - bundle exec bin/gitlab-housekeeper -k Keeps::PrettyUselessKeep -d
+    - bundle exec bin/gitlab-housekeeper -k Keeps::PrettyUselessKeep -d -m 3
+    - bundle exec bin/gitlab-housekeeper -k Keeps::PrettyUselessKeep -d -m 3 --filter-identifiers=2

gems/gitlab-housekeeper/README.md

@@ -90,7 +90,7 @@ module Keeps
         This MR makes a new file #{file_name}
         MARKDOWN
 
-        change.labels = %w(type::feature)
+        change.labels = %w[type::feature]
 
         change.changed_files = [file_name]
 

gems/gitlab-housekeeper/bin/gitlab-housekeeper

 #!/usr/bin/env ruby
 
+$:.unshift File.expand_path("../lib", __dir__)
+
 require "optparse"
 require 'gitlab/housekeeper'
 

gems/gitlab-housekeeper/spec/fixtures/pretty_useless_keep.rb

+# frozen_string_literal: true
+
+module Keeps
+  class PrettyUselessKeep < ::Gitlab::Housekeeper::Keep
+    def each_change
+      (1..3).each do |i|
+        file_name = "new_file#{i}.txt"
+
+        `touch #{file_name}`
+
+        change = ::Gitlab::Housekeeper::Change.new
+
+        change.identifiers = [self.class.name.demodulize, "new_file#{i}"]
+
+        change.title = "Make new file #{file_name}"
+
+        change.description = <<~MARKDOWN
+        ## New files
+
+        This MR makes a new file #{file_name}
+        MARKDOWN
+
+        change.labels = %w[type::feature]
+
+        change.changed_files = [file_name]
+
+        # to push changes without triggering a pipeline.
+        change.push_options.ci_skip = true
+
+        yield(change)
+      end
+    end
+  end
+end