Fatal error during unit testing with MySQL 5.7

When running the search_api_db unit tests against a MySQL 5.7 database, a fatal error is thrown:

Drupal test run
---------------

Tests to be run:
  - Drupal\Tests\search_api_db\Kernel\BackendTest

Test run started:
  Monday, June 6, 2016 - 15:31

Test summary
------------

Drupal\Tests\search_api_db\Kernel\BackendTest                  0 passes   2 fails                            
FATAL Drupal\Tests\search_api_db\Kernel\BackendTest: test runner returned a non-zero error code (2).
Drupal\Tests\search_api_db\Kernel\BackendTest                  0 passes   1 fails                            

Test run duration: 5 sec

Detailed test results
---------------------


---- Drupal\Tests\search_api_db\Kernel\BackendTest ----


Status    Group      Filename          Line Function                            
--------------------------------------------------------------------------------
Fail      Other      BackendTest.php     86 Drupal\Tests\search_api_db\Kernel\B
    Drupal\Tests\search_api_db\Kernel\BackendTest::testFramework
    Drupal\Core\Database\DatabaseExceptionWrapper: SQLSTATE[HY000]: General
    error: 3065 Expression #1 of ORDER BY clause is not in SELECT list,
    references column 'd8.t.id' which is not in SELECT list; this is
    incompatible with DISTINCT: SELECT DISTINCT t.item_id AS item_id, :score AS
    score
    FROM 
    {search_api_db_database_search_index} t
    WHERE (( (t.item_id IN  (SELECT t.item_id AS item_id
    FROM 
    (SELECT t.item_id AS item_id, t.score AS score
    FROM 
    {search_api_db_database_search_index_text} t
    WHERE  (word IN  (:db_condition_placeholder_0)) AND (field_name IN 
    (:db_condition_placeholder_1)) ) t)) OR (t.item_id IN  (SELECT t.item_id AS
    item_id
    FROM 
    (SELECT t.item_id AS item_id, t.score AS score
    FROM 
    {search_api_db_database_search_index_text} t
    WHERE  (word IN  (:db_condition_placeholder_2)) AND (field_name IN 
    (:db_condition_placeholder_3)) ) t)) ))
    ORDER BY t.id ASC
    LIMIT 10 OFFSET 0; Array
    (
        [:db_condition_placeholder_0] => bar
        [:db_condition_placeholder_1] => name
        [:db_condition_placeholder_2] => bar
        [:db_condition_placeholder_3] => body
        [:score] => 1000
    )
    
    
    /Users/vangeiv/Workspace/d8/core/lib/Drupal/Core/Database/Connection.php:671
    /Users/vangeiv/Workspace/d8/core/lib/Drupal/Core/Database/Connection.php:635
    /Users/vangeiv/Workspace/d8/core/lib/Drupal/Core/Database/Driver/mysql/Connection.php:81
    /Users/vangeiv/Workspace/d8/core/lib/Drupal/Core/Database/Query/Select.php:476
    /Users/vangeiv/Workspace/search_api/search_api_db/src/Plugin/search_api/backend/Database.php:1537
    /Users/vangeiv/Workspace/search_api/src/Entity/Server.php:409
    /Users/vangeiv/Workspace/search_api/src/Query/Query.php:376
    /Users/vangeiv/Workspace/search_api/tests/src/Kernel/BackendTestBase.php:301
    /Users/vangeiv/Workspace/search_api/tests/src/Kernel/BackendTestBase.php:94
Fail      run-tests. Unknown              0 Unknown                            
    FATAL Drupal\Tests\search_api_db\Kernel\BackendTest: test runner returned a
    non-zero error code (2).

From the MySQL 5.7 reference manual, 13.20.3 MySQL Handling of GROUP BY:

To prevent this problem, a query that has DISTINCT and ORDER BY is rejected as invalid if any ORDER BY expression does not satisfy at least one of these conditions:

• The expression is equal to one in the select list
• All columns referenced by the expression and belonging to the query's selected tables are elements of the select list

Patch attached checks if the column(s) used for sorting are part of the select list, and adds them if they are not.