selftests: sync: differentiate between sync unsupported and access errors

Sync test doesn't differentiate between sync unsupported and test run
by non-root user and treats both as unsupported cases.

Fix it to add handling for these two different scenarios.

Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>
Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.com>