repatched https://www.drupal.org/project/drupal/issues/1754162#comment-9737269

modules/user/user.test.orig

@@ -321,6 +321,10 @@ class UserLoginTestCase extends DrupalWebTestCase {
     );
   }
 
+  function setUp() {
+    parent::setUp('user_session_test');
+  }
+
   /**
    * Test the global login flood control.
    */
@@ -421,6 +425,17 @@ class UserLoginTestCase extends DrupalWebTestCase {
     $this->assertIdentical(_password_get_count_log2($account->pass), DRUPAL_HASH_COUNT + 1);
   }
 
+  /**
+   * Test logging in when an anon session already exists.
+   */
+  function testLoginWithAnonSession() {
+    // Visit the callback to generate a session for this anon user.
+    $this->drupalGet('user_session_test_anon_session');
+    // Now login.
+    $account = $this->drupalCreateUser(array());
+    $this->drupalLogin($account);
+  }
+
   /**
    * Make an unsuccessful login attempt.
    *