Simple Magento 1 bootstrap for PHPUnit.

bootstrap.php

<?php

ini_set('display_errors', 1);
error_reporting(E_ALL);

require __DIR__ . '/../../../../../../Mage.php';
require __DIR__ . '/util/Magento_IntegrationTest_Config.php';

Mage::setIsDeveloperMode(true);
Mage::app('', 'store', ['config_model' => Magento_IntegrationTest_Config::class]);

$magentoHandler = set_error_handler(function () {
    $usePhpErrorHandlingReturnValue = false;
    return $usePhpErrorHandlingReturnValue;
});

set_error_handler(function ($errno, $errstr, $errfile)  use ($magentoHandler) {
    if (substr($errfile, -19) === 'Varien/Autoload.php') {
        $ignoreErrorReturnValue = null;
        return $ignoreErrorReturnValue;
    }
    return is_callable($magentoHandler) ?
        call_user_func_array($magentoHandler, func_get_args()) :
        false;
});

$_SESSION = [];

Magento_IntegrationTest_Config.php

<?php


class Magento_IntegrationTest_Config extends Mage_Core_Model_Config
{
    private $modelTestDoubles = [];
    private $resourceModelTestDoubles = [];

    public function setModelTestDouble($modelClass, $testDouble)
    {
        $this->modelTestDoubles[$modelClass] = $testDouble;
    }

    public function getModelInstance($modelClass = '', $constructArguments = [])
    {
        if (isset($this->modelTestDoubles[(string) $modelClass])) {
            return $this->modelTestDoubles[(string) $modelClass];
        }
        return parent::getModelInstance($modelClass, $constructArguments);
    }

    public function setResourceModelTestDouble($modelClass, $testDouble)
    {
        $this->resourceModelTestDoubles[$modelClass] = $testDouble;
    }

    public function getResourceModelInstance($modelClass = '', $constructArguments = [])
    {
        if (isset($this->resourceModelTestDoubles[(string) $modelClass])) {
            return $this->resourceModelTestDoubles[(string) $modelClass];
        }
        return parent::getResourceModelInstance($modelClass, $constructArguments);
    }
}