New test to verify correct work of `getAttribute` method

Each of a drivers (due various reasons) had to deal with bugs in underlying libraries. The common problem was, that a missing attribute value was returned as an empty string. To make output of `getAttribute` method consistent a workaround code was added that considered present attributes with empty values as missing ones.

Thus all attributes in following code were considered as missing:
```html
<input disabled/>
<input disabled=""/>
```

I've implemented a series of PR's for all of the drivers to fix things:
* https://github.com/Behat/MinkSelenium2Driver/pull/89
* https://github.com/Behat/MinkSeleniumDriver/pull/18
* https://github.com/Behat/SahiClient/pull/12
* https://github.com/Behat/MinkBrowserKitDriver/pull/32

Since https://github.com/Behat/MinkGoutteDriver and https://github.com/Behat/MinkWUnitDriver are in fact extending https://github.com/Behat/MinkBrowserKitDriver, then no special PR's are needed for them.

This PR concludes work, related to `getAttribute` method fixing by adding a new test to check for all possible use cases.