Ensure orphan tag are remove in API

When the association between a tag and an entry is removed, if the tag doesn’t have other entries, we can remove it.

Also add more tests for that part and ensure TagControllerTest is isolated from the rest of the test suite (finally!)

tests/Wallabag/ApiBundle/Controller/WallabagRestControllerTest.php

@@ -561,6 +561,8 @@ class WallabagRestControllerTest extends WallabagApiTestCase
      */
     public function testDeleteUserTag($tag)
     {
+        $tagName = $tag['label'];
+
         $this->client->request('DELETE', '/api/tags/'.$tag['id'].'.json');
 
         $this->assertEquals(200, $this->client->getResponse()->getStatusCode());
@@ -577,6 +579,13 @@ class WallabagRestControllerTest extends WallabagApiTestCase
             ->findAllByTagId($this->user->getId(), $tag['id']);
 
         $this->assertCount(0, $entries);
+
+        $tag = $this->client->getContainer()
+            ->get('doctrine.orm.entity_manager')
+            ->getRepository('WallabagCoreBundle:Tag')
+            ->findOneByLabel($tagName);
+
+        $this->assertNull($tag, $tagName.' was removed because it begun an orphan tag');
     }
 
     public function testDeleteTagByLabel()