tests: Avoid using dynamic properties

Signed-off-by: Julius Härtl <jus@bitgrid.net>

tests/unit/Db/BoardMapperTest.php

@@ -38,6 +38,7 @@ class BoardMapperTest extends TestCase {
 
 	/** @var IDBConnection */
 	private $dbConnection;
+	private $labelMapper;
 	/** @var AclMapper|\PHPUnit\Framework\MockObject\MockObject */
 	private $aclMapper;
 	/** @var BoardMapper */