Only articles should be used for Posts

An Exception should be thrown if the node passed to the `Post` entity is
not an article.

> Drupal\Tests\my_module\Unit\Entity\PostTest::it_throws_an_exception_if_the_node_is_not_an_article
> Failed asserting that exception of type "InvalidArgumentException" is
thrown.

web/modules/custom/my_module/tests/src/Unit/Entity/PostTest.php

@@ -21,4 +21,15 @@ class PostTest extends UnitTestCase {
     $this->assertSame('Test post', $post->getTitle());
   }
 
+  /** @test */
+  public function it_throws_an_exception_if_the_node_is_not_an_article() {
+    $node = $this->createMock(NodeInterface::class);
+
+    $node->method('bundle')->willReturn('page');
+
+    $this->expectException(\InvalidArgumentException::class);
+
+    new Post($node);
+  }
+
 }