selftests/mm/cow: Add tests for anonymous small-sized THP

Add tests similar to the existing PMD-sized THP tests, but which operate
on memory backed by (PTE-mapped) small-sized THP. This reuses all the
existing infrastructure. If the test suite detects that small-sized THP
is not supported by the kernel, the new tests are skipped.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>