Don't run tests for MinGW 4.7 stages

MinGW 4.7 will be replaced by MinGW 4.8 at any time, so there is no
much point to waste time and resources running these tests.

MinGW 4.7 might actually be removed at any time.

Change-Id: I806b1b3c884046d910ef9ceef74bc6ee5a5ba4ef
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Simo Fält <simo.falt@digia.com>