Add test for running tests in child process which are calling "exit"

tests/_files/SeparateProcessesTest.php

@@ -0,0 +1,29 @@
+<?php declare(strict_types=1);
+/*
+ * This file is part of PHPUnit.
+ *
+ * (c) Sebastian Bergmann <sebastian@phpunit.de>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+use PHPUnit\Framework\TestCase;
+
+/**
+ * @runTestsInSeparateProcesses
+ */
+class SeparateProcessesTest extends TestCase
+{
+    public function testFoo(): void
+    {
+        $this->assertTrue(true);
+        exit(0);
+    }
+
+    public function testBar(): void
+    {
+        $this->assertTrue(true);
+        $this->assertTrue(true);
+        exit(1);
+    }
+}

tests/end-to-end/separate-processes-test.phpt

@@ -0,0 +1,17 @@
+--TEST--
+phpunit --no-configuration ../../_files/SeparateProcessesTest.php
+--FILE--
+<?php declare(strict_types=1);
+$_SERVER['argv'][1] = '--no-configuration';
+$_SERVER['argv'][2] = __DIR__ . '/../_files/SeparateProcessesTest.php';
+
+require __DIR__ . '/../bootstrap.php';
+PHPUnit\TextUI\Command::main();
+--EXPECTF--
+PHPUnit %s by Sebastian Bergmann and contributors.
+
+..                                                                  2 / 2 (100%)
+
+Time: %s, Memory: %s
+
+OK (2 tests, 0 assertions)