Show specific error message when subject not found.

Author: mattstauffer

src/Testing/InteractsWithMail.php

@@ -161,7 +161,10 @@ protected function seeInSubjects($subjects)
         $subjects = (array) $subjects;
 
         foreach ($subjects as $subject) {
-            $this->assertTrue(in_array($subject, $this->getMailer()->subjects()->all()));
+            $this->assertTrue(
+                in_array($subject, $this->getMailer()->subjects()->all()),
+                "Subject not found: `{$subject}`"
+            );
         }
     }