ParserTest: Make sure recursion checking readonly arrays doesn't error

If we assigned a recursion marker to a readonly array we'd get an
error since it's supposed to be immutable. However, since we dump
object properties by first casting instances to arrays it makes a
copy of the array to be checked which _can_ be modified unlike
the property itself.

Now you may be thinking "Doesn't this cause recursion checking to
break for 1 loop when dumping" but it turns out it's impossible to
assign references to readonly properties so this bug exists in the
users code and our recursion check breaking for 1 loop is actually
an accurate representation of what's going on!

tests/Fixtures/Php81TestClass.php

@@ -10,12 +10,16 @@ class Php81TestClass
     public readonly string $a;
     protected readonly string $b;
     private readonly string $c;
+    public readonly array $d;
 
     public function __construct(string $a)
     {
         $this->a = $a;
         $this->b = $a.$a;
         $this->c = $a.$a.$a;
+        $d = [$this->a, [$this->b], [[$this->c]]];
+        $d[] = &$d;
+        $this->d = $d;
     }
 
     public function typeHints(X & Y $p1)

tests/Parser/ParserTest.php

@@ -526,6 +526,8 @@ public function testParseObjectReadonly()
         $this->assertTrue($val[1]->readonly);
         $this->assertSame('c', $val[2]->name);
         $this->assertTrue($val[2]->readonly);
+        $this->assertSame('d', $val[3]->name);
+        $this->assertTrue($val[3]->readonly);
     }
 
     /**