Add E2E tests for login (#64)

* Add E2E test that checks if login form is visible
* Add tests for login where user enters input

See DIH-155

test/features/login.feature

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+
+Feature: Login
+    I want to be able to see the form
+    And I want generic feedback when credentials are missing
+	And I want generic feedback when credentials are wrong
+	And I want to be able to login with correct credentials
+
+    Scenario: Open login page
+        Given I open the page "login"
+        Then I expect that element "email" is visible
+        Then I expect that element "password" is visible
+
+    Scenario: Unsuccessful login with blank input
+        Given I open the page "login"
+        When I press the button "submit"
+        Then I expect that "messages" contains "The combination of username and password is invalid"
+
+    Scenario: Unsuccessful login with invalid credentials
+        Given I open the page "login"
+        When I set "E-mail" to the inputfield "[email protected]"
+        And I set "Password" to the inputfield "wrongpassword"
+        And I press the button "submit"
+        Then I expect that "messages" contains "The combination of username and password is invalid"
+
+    Scenario: Login succeeds with valid credentials
+        Given I open the page "login"
+        When I set "E-mail" to the inputfield "[email protected]"
+        And I set "Password" to the inputfield "password"
+        And I press the button "submit"

test/pages/login.page.js

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+import page from './page';
+
+module.exports = Object.create(page, {
+
+    email: {
+        get() {
+            return browser.element('#email');
+        }
+    },
+
+    password: {
+        get() {
+            return browser.element('#password');
+        }
+    },
+
+    submit: {
+        value() {
+            return browser.click('button#submit');
+        }
+    },
+
+    messages: {
+        get() {
+            return browser.element('li#message');
+        }
+    },
+
+    open: {
+        value() {
+            page.open.call(this, '/login');
+        }
+    }
+});