An error "Fatal error: require(): Failed opening required ..." is raised when launching a test with a relative preload script

```console
$ vendor/bin/testrunner phpunit -p tests/bootstrap.php -R tests

Warning: require(/path/to/project/vendor/phpunit/phpunit-mock-objects/tests/../vendor/autoload.php): failed to open stream: No such file or directory in /path/to/project/vendor/phpunit/phpunit-mock-objects/tests/bootstrap.php on line 4

Call Stack:
    0.0003     271040   1. {main}() /path/to/project/vendor/piece/stagehand-testrunner/bin/testrunner:0
    0.0018     498584   2. Stagehand\TestRunner\Core\preloadScript() /path/to/project/vendor/piece/stagehand-testrunner/bin/testrunner:45
    0.0020     501832   3. include_once('/path/to/project/vendor/phpunit/phpunit-mock-objects/tests/bootstrap.php') /path/to/project/vendor/piece/stagehand-testrunner/bin/testrunner:158


Fatal error: require(): Failed opening required '/path/to/project/vendor/phpunit/phpunit-mock-objects/tests/../vendor/autoload.php' (include_path='/path/to/project/vendor/phake/phake/src:/path/to/project/vendor/phpunit/php-text-template:/path/to/project/vendor/phpunit/phpunit-mock-objects:/path/to/project/vendor/phpunit/php-timer:/path/to/project/vendor/phpunit/php-file-iterator:/path/to/project/vendor/phpunit/php-code-coverage:/path/to/project/vendor/phpunit/phpunit:/path/to/project/vendor/symfony/yaml:.') in /path/to/project/vendor/phpunit/phpunit-mock-objects/tests/bootstrap.php on line 4

Call Stack:
    0.0003     271040   1. {main}() /path/to/project/vendor/piece/stagehand-testrunner/bin/testrunner:0
    0.0018     498584   2. Stagehand\TestRunner\Core\preloadScript() /path/to/project/vendor/piece/stagehand-testrunner/bin/testrunner:45
    0.0020     501832   3. include_once('/path/to/project/vendor/phpunit/phpunit-mock-objects/tests/bootstrap.php') /path/to/project/vendor/piece/stagehand-testrunner/bin/testrunner:158
```