active_model_lint.rb

require "active_model/lint"                                                     
require "test/unit/assertions"                                                  
                                                                                
shared_examples_for "ActiveModel" do                                            
  include ActiveModel::Lint::Tests                                              
  include Test::Unit::Assertions                                                
                                                                                
  before { @model = subject }                                                   
                                                                                
  ActiveModel::Lint::Tests.public_instance_methods.map(&:to_s).grep(/^test/).each do |test|
    example test.gsub("_", " ") do |example|                                    
      send test                                                                 
    end                                                                         
  end                                                                           
end   

errors

  2) User it should behave like ActiveModel test to param
     Failure/Error: send test
     NoMethodError:
       undefined method `assertions' for #<RSpec::ExampleGroups::User::ItShouldBehaveLikeActiveModel:0x007f6d49e30450>
     Shared Example Group: "ActiveModel" called from ./spec/models/user_spec.rb:6
     # ./spec/support/active_model_lint.rb:12:in `block (3 levels) in <top (required)>'

I was running into the same error while upgrading Rails from 4.0 to 4.1. I was able to get around it by adding attr_accessor :assertions and self.assertions = 0 like so:

shared_examples_for 'ActiveModel' do
  ...

  attr_accessor :assertions

  before do
    @model = subject
    self.assertions = 0
  end

  ...
end

Did you come up with a more elegant solution?