selftests: mm: add shmem collapse as a default test item

Currently, we only test anonymous memory collapse by default. We should
also add shmem collapse as a default test item to catch issues that could
break the test cases.

Link: https://lkml.kernel.org/r/a30b1529b399f2e649b5a05c3d352f41a68faeae.1749779183.git.baolin.wang@linux.alibaba.com
Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Reviewed-by: Dev Jain <dev.jain@arm.com>
Tested-by: Dev Jain <dev.jain@arm.com>
Acked-by: David Hildenbrand <david@redhat.com>
Acked-by: Zi Yan <ziy@nvidia.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Mariano Pache <npache@redhat.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>