ToStringPluginTest: Use fixture for bad string test

Turns out kint is so good at detecting stuff that dumping a
phpunit stub will result in an out of memory from dumping the
entirety of phpunit...

tests/Fixtures/BadToStringClass.php

@@ -0,0 +1,13 @@
+<?php
+
+namespace Kint\Test\Fixtures;
+
+use InvalidArgumentException;
+
+class BadToStringClass
+{
+    public function __toString()
+    {
+        throw new InvalidArgumentException('Bad toString');
+    }
+}

tests/Parser/ToStringPluginTest.php

@@ -27,10 +27,9 @@
 
 namespace Kint\Test\Parser;
 
-use InvalidArgumentException;
 use Kint\Parser\Parser;
 use Kint\Parser\ToStringPlugin;
-use Kint\Test\Fixtures\TestClass;
+use Kint\Test\Fixtures\BadToStringClass;
 use Kint\Test\KintTestCase;
 use Kint\Zval\BlobValue;
 use Kint\Zval\Value;
@@ -114,8 +113,7 @@ public function testParseBuggyValue()
         $p->addPlugin(new ToStringPlugin($p));
         $b = new Value('$v');
 
-        $v = $this->createStub(TestClass::class);
-        $v->method('__toString')->willThrowException(new InvalidArgumentException('Bad toString'));
+        $v = new BadToStringClass();
 
         $obj = $p->parse($v, clone $b);