log spec (test) files that run

spec_helper.rb

# RSpec.configure do |config|
  # ...

  $spec_files = Set.new

  config.before(:each) do |example|
    $spec_files << example.metadata[:file_path]
  end

  # keep logging each run spec file in spec_files.txt
  config.after(:all) do
    File.open("spec_files.txt", "w+") do |f|
      f.puts $spec_files.to_a.join(" ")
    end
  end

  # print all spec files in the summary report inside the terminal
  config.after(:suite) do
    puts "Spec files: ", $spec_files.to_a.join(" ")
  end

  # ...
# end