dev: Fix some PHPUnit Testdox warnings

I detected these warnings by running the tests of this specific file.
It appears that silenced warnings are detected by the Testdox output,
but not by the default output. The best would be to never use `@`
anyway.

Reference: sebastianbergmann/phpunit#5884

tests/Service/DataImporter/DataImporterTest.php

@@ -1334,8 +1334,7 @@ public function testImportTicketsImportsDocumentsAsWell(): void
                 ],
             ],
         ];
-        $documentsPath = sys_get_temp_dir() . '/documents';
-        @mkdir($documentsPath);
+        $documentsPath = $this->createDocumentsFolder();
         $content = 'My bug';
         $hash = hash('sha256', $content);
         file_put_contents($documentsPath . '/bug.txt', $content);
@@ -2058,9 +2057,11 @@ public function testImportTicketsFailsIfDocumentIsMissing(): void
                 ],
             ],
         ];
-        $documentsPath = sys_get_temp_dir() . '/documents';
-        @mkdir($documentsPath);
-        @unlink($documentsPath . '/bug.txt');
+        $documentsPath = $this->createDocumentsFolder();
+        $filepath = "{$documentsPath}/bug.txt";
+        if (file_exists($filepath)) {
+            unlink($filepath);
+        }
 
         $this->processGenerator($this->dataImporter->import(
             organizations: $organizations,
@@ -2072,4 +2073,15 @@ public function testImportTicketsFailsIfDocumentIsMissing(): void
         $this->assertSame(0, MessageFactory::count());
         $this->assertSame(0, MessageDocumentFactory::count());
     }
+
+    private function createDocumentsFolder(): string
+    {
+        $documentsPath = sys_get_temp_dir() . '/documents';
+
+        if (!is_dir($documentsPath)) {
+            mkdir($documentsPath);
+        }
+
+        return $documentsPath;
+    }
 }