tests: conditionally skip part of alpha-blending

One of the three fixture setups of alpha-blending test requires
color-lcms.so. If color-lcms.so is not built, that fixture fails.

Make it skip as necessary, making the test suite pass with
color-management-lcms=false build option.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>