Add findChild to TestCase.

This is useful for auto tests where it's necessary to have access to
dynamically created child items. For example:

property Component threeItemDelegate: Text {
    objectName: "delegate" + styleData.index
    text: styleData.value
}

...

function test_stuff() {
    ...

    var delegate0 = findChild(item, "delegate0");
    // check delegate0 for some condition...
}

[ChangeLog][QtTest][TestCase] Added findChild function to TestCase.

Change-Id: I04a8b07c9904768c07ec12f4b03f1afb1989e054
Reviewed-by: Liang Qi <liang.qi@digia.com>
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>