test: Composite with solid images instead of using pixman_image_fill_*

test: Composite with solid images instead of using pixman_image_fill_*

There is a couple of places where the test suite uses the
pixman_image_fill_* functions to initialize images. These functions
can fail, and will do so if the "fast" implementation is disabled.

So to make sure the test suite passes even using
PIXMAN_DISABLE="fast", use pixman_image_composite32() with a solid
image instead of pixman_image_fill_*.