Added regression test

Author: staabm

tests/end-to-end/regression/5993.phpt

@@ -0,0 +1,14 @@
+--TEST--
+https://github.com/sebastianbergmann/phpunit/issues/5993
+--FILE--
+<?php declare(strict_types=1);
+$_SERVER['argv'][] = '--do-not-cache-result';
+$_SERVER['argv'][] = '--no-configuration';
+$_SERVER['argv'][] = '--debug';
+$_SERVER['argv'][] = __DIR__ . '/5993/Issue5993Test.php';
+
+require_once __DIR__ . '/../../bootstrap.php';
+
+(new PHPUnit\TextUI\Application)->run($_SERVER['argv']);
+--EXPECTF--
+%A

tests/end-to-end/regression/5993/Issue5993Test.php

@@ -0,0 +1,29 @@
+--TEST--
+PHPUnit process is blocked when there's a lot of output and a test with separate process
+--INI--
+error_reporting=-1
+display_errors=1
+display_startup_errors=1
+memory_limit=-1
+zend.assertions=1
+assert.exception=1
+--SKIPIF--
+<?php
+for ($i = 0; $i < 390; $i++) {
+    trigger_error("error $i");
+}
+?>
+--FILE--
+<?php
+
+use PHPUnit\Framework\Attributes\RunInSeparateProcess;
+use PHPUnit\Framework\TestCase;
+
+final class Issue5993Test extends TestCase
+{
+    #[RunInSeparateProcess]
+    public function testOne(): void
+    {
+        $this->assertTrue(true);
+    }
+}