test: Fix stride calculation in stress-test

Currently the number of bits per pixel is used instead of the
number of bytes per pixel when calculating image strides. This
does not cause any real problems, but the gaps between scanlines
are excessively large.

This patch actually converts bits to bytes and rounds up the result
to the nearest byte boundary.

Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>
Reviewed-by:  <soren.sandmann@gmail.com>