MAINTAINERS: add missing test files to mm gup section

We previously overlooked GUP test files that sensibly should belong to the
GUP section, include them now.

Link: https://lkml.kernel.org/r/20250616200844.560225-1-lorenzo.stoakes@oracle.com
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Acked-by: David Hildenbrand <david@redhat.com>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Peter Xu <peterx@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: ljskernel

MAINTAINERS

@@ -15795,6 +15795,10 @@ S:	Maintained
 W:	http://www.linux-mm.org
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
 F:	mm/gup.c
+F:	mm/gup_test.c
+F:	mm/gup_test.h
+F:	tools/testing/selftests/mm/gup_longterm.c
+F:	tools/testing/selftests/mm/gup_test.c
 
 MEMORY MANAGEMENT - KSM (Kernel Samepage Merging)
 M:	Andrew Morton <akpm@linux-foundation.org>