tests: update activeFocusOnTab to support editable ComboBox

ComboBox was editable since 4c184095d378622fb0c3bb1692b331b3cc04e010. It
should get focus when editable is true in tab focus chain. The behavior
was fixed in qtdeclarative side.

Change-Id: I85f26e99e00f970b8603d8ca23b1e55daa52159b
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>