Add test_baseUrlAfterLoadHtml QML test case

Remove unreachableUrl support from WebContentsAdapter::setContent,
because this argument behaves exactly the same as baseUrl.

Change-Id: I36f92b99b7045c6d3b831481bb04d51a0e05772f
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>