application_helper.rb

module ApplicationHelper
  def test_asset_path
    asset_path nil
  end
end

application_helper_test.rb

require 'test_helper'

class ApplicationHelperTest < ActionView::TestCase
  def test_path
    assert_equal '/assets/', test_asset_path
  end
end

output

Run options: 

# Running tests:

E

Finished tests in 0.177110s, 5.6462 tests/s, 0.0000 assertions/s.

  1) Error:
test_path(ApplicationHelperTest):
NoMethodError: undefined method `asset_path' for #<ApplicationHelperTest:0x007fd05d876c90>
    /Users/bemathis/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/actionpack-3.2.8/lib/action_dispatch/testing/assertions/routing.rb:176:in `method_missing'
    /Users/bemathis/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/actionpack-3.2.8/lib/action_view/test_case.rb:236:in `method_missing'
    /Users/bemathis/Dev/Ruby/issues/issue_7534/app/helpers/application_helper.rb:3:in `test_asset_path'
    /Users/bemathis/Dev/Ruby/issues/issue_7534/test/unit/application_helper_test.rb:5:in `test_path'

1 tests, 0 assertions, 0 failures, 1 errors, 0 skips
Errors running test:units! #<RuntimeError: Command failed with status (1): [/Users/bemathis/.rbenv/versions/1.9.3-p194...]>