selftests/mm/cow: Add tests for small-order anon THP

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

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