Fix BrowserStack tests

[shawnbuso]

The BrowserStack tests are super flaky and fail at random with every run. They're currently disabled - let's fix them and re-enable them.

[mysuf]

One recommendation based on my experiences:
I had to add player.ready to tests as videojs is growing and it takes too long to load and properly handle webdriver click (with v6.10.0 firefox always failed in random tests). This fixed it for me:

ads.js

player.ready(function() {
  var log = document.getElementById('log');
  log.innerHTML += "ready<br>"; 
});

basic.js:

let log = await driver.findElement(By.id('log'));
await driver.wait(until.elementTextContains(log, 'ready'), 10000);
await driver.findElement(By.id('content_video')).click();
....

[mysuf]

You can also try newer version of geckodriver, but looking at browserstack waiting times(across both drivers), this seems more like really poor browserstack HW/connection capabilities..

[ypavlotsky]

Fixed with #657