sanitycheck: sort tests by platform when subsetting

This undo's commit dd65f7c38a5 that changed how we sorted tests to get
better distribution from a qemu running point of view.  However we now
have some tests that take a long time to build and we end up loading
build machines we all the long running tests.

Lets try going back to sorting by platform and see how that behaves with
the new CI environment.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>
1 file changed