mountinfo/TestMountedBy*: add missing pre-checks
mountinfo/TestMountedBy*: add missing pre-checks

Both tests need (1) root (2) openat2.

Do skip if those are not available.

Signed-off-by: Kir Kolyshkin <3a017b8ddb3f9cf3e4a59978b004111bdeb97f08@gmail.com>

moby/sys