Add ReadmeTestHelper

This helper provides utilities for extracting code snippets from
`README.md`.

Author: sambostock

test/readme_test_helper.rb

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+module ReadmeTestHelper
+  private
+
+  # Extracts a code snippet from the README.md file by its "SNIPPET ID" comment and language
+  def extract_readme_code_snippet(id, language: "ruby")
+    snippet = readme_content[/<!-- SNIPPET ID: #{id} -->\n```#{language}\n(.*?)\n```/m, 1]
+    assert_not_nil(snippet, "Could not find code snippet with ID #{id}")
+    snippet
+  end
+
+  def readme_content = File.read(File.join(project_root, "README.md"))
+  def project_root = File.expand_path("..", __dir__)
+end