Coverage: Don't attempt to read cookies while an alert is open

[marcovtwout]

Fixes #1485

Please check and test this workaround thoroughly.

[Naktibalda]

Wouldn't it be better to catch exception thrown by grabCookie?

[marcovtwout]

@Naktibalda It's actually not grabCookie() that throws the exception. Calling grabCookie() (which internally calls WebDriver->manage()->getCookies()) unexpectedly dismisses the alert and execution continues. Then the next call in your acceptance tests such as $I->acceptPopup() will fail since the alert is no longer present.