Fix form edition in webdriver tests using Mac OS

Some of our helpers in webdriver tests are not working correctly when using MacOS, instead of editing an input text field after having cleared it, it adds up new content to it.

Works ok under linux or windows, as the edition code is actually working around an old bug on "clear" method by using key CTRL+a before inserting new content, this was setup as a workaround for NXP-16465.