tools/testing/selftests: extend mremap_test to test multi-VMA mremap

tools/testing/selftests: extend mremap_test to test multi-VMA mremap

Now that we have added the ability to move multiple VMAs at once, assert
that this functions correctly, both overwriting VMAs and moving backwards
and forwards with merge and VMA invalidation.

Additionally assert that page tables are correctly propagated by setting
random data and reading it back.

Link: https://lkml.kernel.org/r/139074a24a011ca4ed52498a7fa2080024b43917.1752770784.git.lorenzo.stoakes@oracle.com
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Jann Horn <jannh@google.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>