Doc:Updated link to Qt Test in the essential modules page.
Doc:Updated link to Qt Test in the essential modules page.

Qt Test now has a landing page.

Change-Id: I7a6ab39686595bbfdd2ed08c5ffb0730532590f0
Reviewed-by: Geir Vattekar <566b604d287edf88fa7389d1cb9015d99d71f27e@digia.com>

kobolabs/qtdoc,kobolabs/qtdoc,kobolabs/qtdoc,kobolabs/qtdoc