selftests/mm/cow: Add large anon folio tests

Add tests similar to the existing THP tests, but which operate on memory
backed by large anonymous folios, which are smaller than THP.

This reuses all the existing infrastructure. If the test suite detects
that large anonyomous folios are not supported by the kernel, the new
tests are skipped.

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