Testing: Update MockTarget for consistency with StreamTarget

- Add `MockTarget::__destruct()`

Author: lkrms

src/Toolkit/Testing/Console/MockTarget.php

@@ -45,6 +45,14 @@ public function __construct(
         $this->Formatter = $formatter ?? new Formatter(null, null, fn() => $this->Width);
     }
 
+    /**
+     * @internal
+     */
+    public function __destruct()
+    {
+        $this->close();
+    }
+
     /**
      * @inheritDoc
      */
@@ -135,10 +143,7 @@ public function close(): void
         $this->IsStdout = false;
         $this->IsStderr = false;
         $this->IsTty = false;
-        $this->Width = null;
         $this->Stream = null;
-        unset($this->Formatter);
-
         $this->IsValid = false;
     }