Support multiple A/B tests at once

[grimmdude]

One limitation of this library is that only a single A/B test can be run at once. This is because whenever a page view is counted it cycles through all available tests defined in config/ab-testing.php.

I think we can support multiple tests by making some changes so that we can pass an array of variants as a second argument into the Abtest::isExperiment() method. For instance, something like this:

if (Abtest::isExperiment('booking-page-a', ['booking-page-a', 'booking-page-b'] )) {
    $test = 'a';
}