INT-16757: tearDown() fix for unit tests and removed files from phpunit

Author: jgg9026

tests/mail_test.php

@@ -63,7 +63,7 @@ public function setUp(): void {
         $this->helper = $helper;
     }
 
-    public function tearDown() {
+    public function tearDown(): void {
         $this->helper->messagesink->clear();
         $this->helper->messagesink->close();
 

tests/maildigest_test.php

@@ -90,7 +90,7 @@ public function setUp(): void {
     /**
      * Clear the message sinks set up in this test.
      */
-    public function tearDown() {
+    public function tearDown(): void {
         $this->helper->messagesink->clear();
         $this->helper->messagesink->close();