phpunit.xml

<phpunit>
    <php>
        <env name="BROWSER" value="firefox"/>
    </php>
</phpunit>

TestCase1.php

<?php

class TestCase extends PHPUnit_Framework_TestCase
{
    private $browser;

    protected function setUp()
    {
        $this->browser = new Browser('firefox');
    }

    // ...
}

TestCase2.php

<?php

class TestCase extends PHPUnit_Framework_TestCase
{
    private $browser;

    protected function setUp()
    {
        $this->browser = new Browser(getenv('BROWSER'));
    }

    // ...
}