Include `textarea` element in `testAdvancedForm` test

The `setValue` and `getValue` methods are too complex in some drivers (e.g. in MinkZombieDriver), that `testAdvancedForm` exists to verify their correct behavior. It appears though `textarea` element isn't tested at all.

The problem was originally discovered in #215 issue (related to MinkSelenium2Driver) and test was added specifically for that driver instead of updating `testAdvancedForm`.