selftests: add mincore() tests

Add a test suite for the mincore() syscall. It tests most of its use
cases as well as its interface.

Tests implemented:

- basic interface test
- behavior on anonymous mappings
- behavior on anonymous mappings with huge tlb pages
- file-backed mapping with a regular file
- file-backed mapping with a tmpfs file

Signed-off-by: Ricardo Cañuelo <ricardo.canuelo@collabora.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/20200728100450.4065-1-ricardo.canuelo@collabora.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>