selftests/mm: add messages about test errors to the cow tests

selftests/mm: add messages about test errors to the cow tests

It is not sufficiently clear what the individual tests in the cow test
program are checking so add messages for the failure cases.

Link: https://lkml.kernel.org/r/20250610-selftest-mm-cow-tweaks-v1-4-43cd7457500f@kernel.org
Signed-off-by: Mark Brown <broonie@kernel.org>
Suggested-by: David Hildenbrand <david@redhat.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>