tools/testing/selftests: explicitly test split multi VMA mremap move

Check that moving a range of VMAs where we are offset into the first and
last VMAs works correctly.

This results in the VMAs being split at these points at which we are offset
into VMAs.

We explicitly test both the ordinary MREMAP_FIXED multi VMA move case and
the MREMAP_DONTUNMAP multi VMA move case.

Link: https://lkml.kernel.org/r/b04920bb6c09dc86c207c251eab8ec670fbbcaef.1753119043.git.lorenzo.stoakes@oracle.com


Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Jann Horn <jannh@google.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>