Use Bamboo-provided envvars to set self::$browsers

This functionality requires Bamboo Sauce OnDemand Plugin 1.6.33 or later.

To the best of my knowledge, PHP doesn't have a magic function like __construct that fires when a class is loaded, so I'm forced to call

MyTest::setUpSauceOnDemandBrowsers();

at the bottom of each of my *Test.php files.