InvalidVariableAssignRule: test null coalesce assign (phpstan#5277)

Author: staabm

tests/PHPStan/Rules/Operators/data/bug-3585.php

@@ -22,3 +22,9 @@ function baz(): void
 {
 	$this = 1; // PHP forbids this too
 }
+
+class Test {
+	public function foobar():void {
+		$this ??= 123;
+	}
+}

tests/PHPStan/Rules/Variables/InvalidVariableAssignRuleTest.php

@@ -43,6 +43,10 @@ public function testBug3585(): void
 				'Cannot re-assign $this.',
 				23,
 			],
+			[
+				'Cannot re-assign $this.',
+				28,
+			],
 		]);
 	}