Add feature "Sign in"

features/sign_in.feature

@@ -0,0 +1,12 @@
+Feature: Sign in
+  Background:
+    Given I visit "/users/sign_in"
+
+  Scenario:
+    When I fill form with the following data:
+      | key    | value            |
+      | Email  | user@example.com |
+      | Пароль | q1w2e3r4t5y6     |
+    And I click the form button "Войти"
+    Then I see main page
+    And I am logged in as "user@example.com"

features/step_definitions/stepdefs.rb

@@ -18,6 +18,10 @@ When 'I click the form button {string}' do |string|
   end
 end
 
+Then 'I am logged in as {string}' do |string|
+  expect(page).to have_css 'span.navbar-text', text: string
+end
+
 Then 'I see main page' do
   expect(page).to have_css(
     'h1',