selftests/mm: report unique test names for each cow test

The kselftest framework uses the string logged when a test result is
reported as the unique identifier for a test, using it to track test
results between runs.  The cow test completely fails to follow this
pattern, it runs test functions repeatedly with various parameters with
each result report from those functions being a string logging an error
message which is fixed between runs.

Since the code already logs each test uniquely before it starts refactor
to also print this to a buffer, then use that name as the test result. 
This isn't especially pretty but is relatively straightforward and is a
great help to tooling.

Link: https://lkml.kernel.org/r/20250527-selftests-mm-cow-dedupe-v2-3-ff198df8e38e@kernel.org


Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>