Testconfig: Adding MinGW 4.8 for Qt 4.8

Adding new stage for Qt 4.8.x. Stage is forced to succeed as well
as ignoring test results, until passing in CI.

Change-Id: Ia2bc4a333f68fab98341594b1c98785f132ce544
Reviewed-by: Tony Sarajärvi <tony.sarajarvi@digia.com>
Reviewed-by: Sergio Ahumada <sahumada@blackberry.com>
Reviewed-by: Akseli Salovaara <akseli.salovaara@digia.com>