minor #47193 [Notifier] Fix test logic (fabpot)

This PR was merged into the 5.4 branch.

Discussion
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[Notifier] Fix test logic

| Q             | A
| ------------- | ---
| Branch?       | 5.4
| Bug fix?      | no
| New feature?  | no <!-- please update src/**/CHANGELOG.md files -->
| Deprecations? | no <!-- please update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Tickets       | n/a <!-- prefix each issue number with "Fix #", no need to create an issue if none exist, explain below instead -->
| License       | MIT
| Doc PR        | n/a

This does not change anything, but the test logic is wrong here.

Commits
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a03c1e5fca [Notifier] Fix test logic

Tests/FakeSmsEmailTransportTest.php

@@ -97,6 +97,6 @@ public function testSendWithCustomTransport()
         $this->assertSame(sprintf('New SMS on phone number: %s', $phone), $sentEmail->getSubject());
         $this->assertSame($subject, $sentEmail->getTextBody());
         $this->assertTrue($sentEmail->getHeaders()->has('X-Transport'));
-        $this->assertSame($transportName, $sentEmail->getHeaders()->get('X-Transport')->getBodyAsString());
+        $this->assertSame($transportName, $sentEmail->getHeaders()->get('X-Transport')->getBody());
     }
 }