tests: Test huge page allocation failure.

Our implementation of operate() will try to fill as much as the address space
as possible with huge pages. If that fails, we should be able to fill the range
with small pages instead of failing. This test should make sure that in such
scenarios, the resulting mapping looks sane.

Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>