tests/etnaviv_2d_test: check whether the rendering is correct

Instead of always dumping the rendered picture, check whether it matches
the expectations. This makes more sense for automated testing.

Retain the ability to dump the picture instead of checking it when a
file name is given as an argument. This also removes use of a hardcoded
file name in a world writable directory, which is an unsafe thing to
do anyway.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>