Use Pest instead of PHPT files

Although I love PHPT files, and I've done my fair share of making it
easier to write them in this library, they're very slow, and running
them has become a hindrance.

I've been fidgeting with the idea of using Pest for a while, and I think
it's the right tool for the job. I had to create a couple of functions
to make it easier to run those tests, and now they're working really
alright.

I migrated all the PHPT files into Pest files -- I automated most of the
work with a little script using "nikic/php-parser"; this commit should
contain all the previous PHPT tests as Pest tests.

The previous integration tests would take sixteen seconds, and the Pest
tests take less than a second.

tests/feature/NotWithoutRecursionTest.php

@@ -0,0 +1,18 @@
+<?php
+
+/*
+ * Copyright (c) Alexandre Gomes Gaigalas <alganet@gmail.com>
+ * SPDX-License-Identifier: MIT
+ */
+
+declare(strict_types=1);
+
+use Respect\Validation\Validator;
+
+test('Scenario #1', expectMessage(
+    function (): void {
+        $validator = Validator::not(Validator::intVal()->positive());
+        $validator->assert(2);
+    },
+    '2 must not be an integer value',
+));