PHPUnit Hides Errors After set_error_handler() Called

CustomLogger.php

<?php

class CustomLogger {
    public function __construct() {
        // Try commenting out this line:
        set_error_handler(array($this, 'handleError'));
    }

    public function __desctruct() {
        restore_error_handler();
    }

    public function handleError($errno, $errstr, $errfile = null, $errline = null, $errcontext = null) {
        // Do stuff.
        // Log something.
        return true;
    }
}

CustomLoggerTest.php

<?php
/**
 * CustomLogger, Thing, CustomLoggerTest, ThingTest
 *
 * Place all 4 files in the same directory. Run `phpunit .`. No errors will be shown.
 * Now run `phpunit --process-isolation .`. Errors will be shown.
 * Also try `phpunit ThingTest.php`. Errors will be shown.
 * Tested with PHPUnit 5.7.15.
 */

require_once "CustomLogger.php";

class CustomLoggerTest extends PHPUnit_Framework_TestCase {
    public function testSomething() {
        $logger = new CustomLogger();

        $this->assertTrue(true);
    }

    public function testSomethingElse() {
        $logger = new CustomLogger();

        $this->assertTrue(true);
    }
}

Thing.php

<?php

class Thing {
    public function doStuff() {
        // Do stuff.
    }
}

ThingTest.php

<?php

require_once "Thing.php";

class ThingTest extends PHPUnit_Framework_TestCase {
    public function testTheThing() {
        array_keys(10); // Throw an error!

        $this->assertTrue(true);
    }
}