List Qt Quick Test as an essential module

Change-Id: I1101b500b7ace7b2f79adf363fc65815796fafb4
Reviewed-by: Venugopal Shivashankar <Venugopal.Shivashankar@qt.io>

doc/src/qtmodules.qdoc

@@ -85,6 +85,10 @@
         \li \l[QtQuickLayouts]{Qt Quick Layouts}
         \li Layouts are items that are used to arrange Qt Quick 2 based items in
             the user interface.
+    \row
+        \li \l[QtQuickTest]{Qt Quick Test}
+        \li A unit test framework for QML applications, where the test cases
+            are written as JavaScript functions.
     \row
         \li \l[QtSql]{Qt SQL}
         \li Classes for database integration using SQL.