Split the test suite using RSpec tags.

Rakefile

# Rakefile

begin
  require 'rspec/core/rake_task'

  desc 'Provide private interfaces documentation'
  RSpec::Core::RakeTask.new(:spec)

  namespace :spec do
    desc 'Provide public interfaces documentation'
    RSpec::Core::RakeTask.new(:public) do |t|
      # see https://www.relishapp.com/rspec/rspec-core/v/3-2/docs/command-line/tag-option
      t.rspec_opts = "--tag public"
    end
  end

  namespace :spec do
    desc 'Provide private interfaces documentation for development purpose'
    RSpec::Core::RakeTask.new(:development) do |t|
      # see https://www.relishapp.com/rspec/rspec-core/v/3-2/docs/command-line/tag-option
      t.rspec_opts = "--tag protected --tag private"
    end
  end
rescue LoadError
  desc 'RSpec rake task not available'
  task :spec do
  abort 'RSpec rake task is not available. Be sure to install rspec-core as a gem or plugin'
  end
end

# ...

# task :default => ['spec:public', 'spec:development']