How to unit test module methods

test_a.rb

require 'test/unit'
$VERBOSE = true

#require 'a'
module A
  extend self
  def add(x, y)
    x + y
  end
end
#---

class Test_A < Test::Unit::TestCase
  def test_add
    assert_equal( A.add(3, 1), 4 )
  end
end

zzz_output.log

$ ruby test_a.rb 
Loaded suite flarv
Started
.

Finished in 0.001174862 seconds.
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
1 tests, 1 assertions, 0 failures, 0 errors, 0 pendings, 0 omissions, 0 notifications
100% passed
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
851.16 tests/s, 851.16 assertions/s
$