selftests/mm: increase timeout from 180 to 900 seconds

The mm selftests are timing out with the current 180-second limit.
Testing shows that run_vmtests.sh takes approximately 11 minutes
(664 seconds) to complete.

Increase the timeout to 900 seconds (15 minutes) to provide sufficient
buffer for the tests to complete successfully.

Link: https://lkml.kernel.org/r/20250609120606.73145-2-shivankg@amd.com


Signed-off-by: Shivank Garg <shivankg@amd.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>