Attempt to fix random failures in frontend tests by not setting implicitlyWait to zero at the end of tests

I was looking into the random failure on Travis (see below). 

It looks like the web driver is seeing the setting for 0, and scheduling it on its event loop - this may then be run just after `driver.quit()` has been called in after, hence causing the random fail.

```
Error: This driver instance does not have a valid session ID (did you call WebDriver.quit()?) and may no longer be used.
at checkHasNotQuit (/home/travis/build/mozilla/talkilla/node_modules/selenium-webdriver/lib/webdriver/webdriver.js:276:13)
at flow.execute.then.value (/home/travis/build/mozilla/talkilla/node_modules/selenium-webdriver/lib/webdriver/webdriver.js:250:5)
at webdriver.promise.ControlFlow.runInNewFrame_ (/home/travis/build/mozilla/talkilla/node_modules/selenium-webdriver/lib/webdriver/promise.js:1431:20)
at webdriver.promise.ControlFlow.runEventLoop_ (/home/travis/build/mozilla/talkilla/node_modules/selenium-webdriver/lib/webdriver/promise.js:1296:8)
at Timer.exports.setInterval.timer.ontimeout (timers.js:234:14)

==== async task ====
WebDriver.manage().timeouts().implicitlyWait(0)
at webdriver.WebDriver.schedule (/home/travis/build/mozilla/talkilla/node_modules/selenium-webdriver/lib/webdriver/webdriver.js:245:15)
at webdriver.WebDriver.Timeouts.implicitlyWait (/home/travis/build/mozilla/talkilla/node_modules/selenium-webdriver/lib/webdriver/webdriver.js:1095:23)
```