[BUGFIX] Fix FileSpoolTest creating "spool" directory on root level

The FileSpoolTest is creating a directory "spool" on root level.
Unit tests should never create directories on the root level of
TYPO3 project. Instead use `Environment::getVarPath()` as path.

Resolves: #94858
Releases: master,10.4
Change-Id: I04d1aa246d496b175361f22aa49ed2ac8fec3853
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/70520


Reviewed-by: Simon Gilli <typo3@gilbertsoft.org>
Reviewed-by: Rudy Gnodde <rudy@famouswolf.com>
Reviewed-by: Marcin Sągol <marcin@soee.pl>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Simon Gilli <typo3@gilbertsoft.org>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Marcin Sągol <marcin@soee.pl>
Tested-by: Benni Mack <benni@typo3.org>

typo3/sysext/core/Tests/Unit/Mail/FileSpoolTest.php

@@ -21,6 +21,7 @@ use Symfony\Component\Mailer\Envelope;
 use Symfony\Component\Mailer\Transport\NullTransport;
 use Symfony\Component\Mime\Address;
 use Symfony\Component\Mime\RawMessage;
+use TYPO3\CMS\Core\Core\Environment;
 use TYPO3\CMS\Core\Mail\FileSpool;
 use TYPO3\TestingFramework\Core\Unit\UnitTestCase;
 
@@ -45,7 +46,7 @@ class FileSpoolTest extends UnitTestCase
     protected function setUp(): void
     {
         parent::setUp();
-        $this->subject = new FileSpool('./spool');
+        $this->subject = new FileSpool(Environment::getVarPath() . '/spool/');
         $this->subject->setMessageLimit(10);
         $this->subject->setTimeLimit(1);
     }