use etna_viv/src/test2d/clear2d.c as basis for this test

Signed-off-by: Christian Gmeiner <christian.gmeiner@gmail.com>
fix creation of cmd stream

This took me ages to find out. etna_context_reloc(..) adds
the placeholder word by itselfe.

Signed-off-by: Christian Gmeiner <christian.gmeiner@gmail.com>