Add a test that "apitrace diff-images" flags mismatching images.

[apitrace-tests] / cli / cli-diff-images-mismatch.script

rm_and_mkdir ./tri-out
apitrace dump-images -o ./tri-out/tri tri.trace
EXPECT_FAILURE: apitrace diff-images -v ./tri-ref-mismatch ./tri-out
expect "Comparing ./tri-ref-mismatch/tri0000000027.png and ./tri-out/tri0000000027.png ... MISMATCH\n"