example_test.rb

# test/integration/example_test.rb

require "test_helper"

class ExampleTest < ActionController::IntegrationTest

  setup do
    Admin.create(:email => '[email protected]', :password => 'password')
  end

  test "admin sign in" do
    visit admin_root_path
    assert_equal current_path, new_admin_session_path

    fill_in 'admin_email', :with => '[email protected]'
    fill_in 'admin_password', :with => 'invalid'
    click_button 'admin_submit'
    assert page.has_content?('Invalid email or password')
    assert_equal current_path, new_admin_session_path

    fill_in 'admin_email', :with => '[email protected]'
    fill_in 'admin_password', :with => 'password'
    click_button 'admin_submit'
    assert page.has_content?('Signed in successfully')
    assert_equal current_path, admin_root_path
  end

end

Gemfile

# Gemfile

source 'http://rubygems.org'

gem 'rails'

group :test do
  gem 'capybara'
  gem 'launchy'
end

test_helper.rb

# test/test_helper.rb

ENV["RAILS_ENV"] = "test"
require File.expand_path('../../config/environment', __FILE__)
require 'rails/test_help'
require 'capybara/rails'

class ActiveSupport::TestCase
  setup do
    Capybara.reset_sessions!
  end
end

class ActionController::IntegrationTest
  include Capybara
end

This is interesting.

You write:

click_button 'admin_submit'
assert page.has_content?('Invalid email or password')
assert_equal current_path, new_admin_session_path

current_path matches new_admin_session_path, no problem here.

Instead if you write:

click_button 'admin_submit'
assert_equal current_path, new_admin_session_path
assert page.has_content?('Invalid email or password')

Then current_path does not match new_admin_session_path
I think it is because the page is not loaded fast enough for Capybara.
Too bad as the latter style is more logical.