tests: x86/pagetables: skip table dumping if large memory

For boards with (relatively) large memory, the test which dumps
page tables takes a long time to finish. The default timeout of
sanitycheck is not enough for those boards. UP Squared board is
such a board. So limits to pagetable dumping to boards with
less than 32MB.

Fixes #28548

Signed-off-by: Daniel Leung <daniel.leung@intel.com>