assertions.rb

module Assertions
  module_function

  def assert_close_enough value1, value2
    assert_equal value1.round, value2.round  
  end
end

other_thing_test.rb

require_relative 'test_helper'

class TestOtherThing < MiniTest::Unit::TestCase
  def test_closeness
    Assertions.assert_close_enough 1.7, 1.8
  end
end

test_helper.rb

require_relative 'assertions'

thing_test.rb

require_relative 'test_helper'

class TestThing < MiniTest::Unit::TestCase
  include Assertions
  
  def test_closeness
    assert_close_enough 1.5, 1.6
  end
end