Use convertPostToTweet within the test

Use the convertPostToTweet action within the tests rather than calling
the toTweet() method, and delegate the Action to using the toTweet()
functionality.

web/modules/custom/blog/src/Action/ConvertPostToTweet.php

@@ -8,4 +8,8 @@ use Drupal\opdavies_blog\Entity\Node\Post;
 
 final class ConvertPostToTweet {
 
+  public function __invoke(Post $post): string {
+    return $post->toTweet();
+  }
+
 }

web/modules/custom/blog/tests/src/Kernel/Entity/Node/PostTest.php

@@ -59,7 +59,7 @@ final class PostTest extends EntityKernelTestBase {
     #AutomatedTesting #DDEV #Drupal #Drupal8 #PHP
     EOF;
 
-    $this->assertSame($expected, $post->toTweet());
+    $this->assertSame($expected, ($this->convertPostToTweet)($post));
   }
 
   /** @test */
@@ -79,7 +79,7 @@ final class PostTest extends EntityKernelTestBase {
     #Drupal #PHP
     EOF;
 
-    $this->assertSame($expected, $post->toTweet());
+    $this->assertSame($expected, ($this->convertPostToTweet)($post));
   }
 
   protected function setUp() {