move the diff_test module to tests/modules/diff_test

Done. Just moved the files.

Here's the updated patch.

Well instead of deleting and then creating the file, if you just do the move, that will make patch lighter and more readable.

Here i moved the files with git mv command.

Still deleting and then re-creating the file. By move, I mean patch should mention that file has been moved.

It looks like that block module needs to be added as a dependency:

exception: [Uncaught exception] Line 130 of core/lib/Drupal/Core/Entity/EntityTypeManager.php:
Drupal\Component\Plugin\Exception\PluginNotFoundException: The "block" entity type does not exist. in Drupal\Core\Entity\EntityTypeManager->getDefinition() (line 130 of /var/www/html/core/lib/Drupal/Core/Entity/EntityTypeManager.php). Drupal\Core\Entity\EntityTypeManager->getDefinition('block')
Drupal\Core\Entity\EntityTypeManager->getHandler('block', 'storage')
Drupal\Core\Entity\EntityTypeManager->getStorage('block')
Drupal\Core\Entity\EntityManager->getStorage('block')
entity_create('block', Array)
Drupal\simpletest\WebTestBase->drupalPlaceBlock('local_tasks_block')
Drupal\diff\Tests\DiffRevisionTest->testRevisionDiffOverview()
Drupal\simpletest\TestBase->run(Array)
simpletest_script_run_one_test('1', 'Drupal\diff\Tests\DiffRevisionTest')

+dependencies:
+ - diff
+ - block

Which could magically resolve the other error (fingers crossed)

fail: [Other] Line 728 of core/scripts/run-tests.sh:
Unable to install modules node, diff, diff_test, block due to missing modules diff_test.

The test were failing because the test module was moved in the wrong directory. In fact the module needs to be moved outside of the module namspace i.e /src folder. Basically in the module root folder a new /tests folder is created and the module moved there.