Add test for unset.

Author: mad-briller

tests/PHPStan/Rules/Properties/MissingReadOnlyPropertyAssignRuleTest.php

@@ -52,7 +52,7 @@ public function testBug13856(): void
 		$this->analyse([__DIR__ . '/data/bug-13856.php'], [
 			[
 				'Readonly property Bug13856\foo2::$store is already assigned.',
-				27,
+				28,
 			],
 		]);
 	}

tests/PHPStan/Rules/Properties/data/bug-13856.php

@@ -13,6 +13,7 @@ public function __construct()
     {
         $this->store = new SplObjectStorage();
         $this->store[(object) ['foo' => 'bar']] = true;
+        unset($this->store[(object) ['foo' => 'bar']]);
     }
 }