Test cases nesting for test-unit gem like a describe/context nesting of RSpec

test_a.rb

# At first, run `gem install test-unit`.

require 'test/unit'

class TestA < Test::Unit::TestCase
  setup { puts "all setup" }

  test("a") { assert true }

  sub_test_case "a sub" do
    setup { puts "a sub setup" }

    sub_test_case "a sub sub1" do
      setup { puts "a sub sub1 setup" }
      test("a sub sub1") { assert true }
    end

    sub_test_case "a sub sub2" do
      setup { puts "a sub sub2 setup" }
      test("a sub sub2") { assert true }
    end
  end
end

#=>
# Loaded suite test_a
# Started
# all setup
# .all setup
# a sub setup
# a sub sub1 setup
# .all setup
# a sub setup
# a sub sub2 setup
# .
# 
# Finished in 0.00419507 seconds.
# ----------------------------------------------------------------------------------------------------------------------------------------
# 3 tests, 3 assertions, 0 failures, 0 errors, 0 pendings, 0 omissions, 0 notifications
# 100% passed
# ----------------------------------------------------------------------------------------------------------------------------------------
# 715.13 tests/s, 715.13 assertions/s