selftest: check file readability in shadow_copy2 test

selftest: check file readability in shadow_copy2 test

Add tests which verify that a snapshot file is readable
if and only if it its metadata can be retrieved. Also
verify (in most tests) that file is retrieved from the
correct snapshot.

Together with the existing test for number of previous
versions we can stat, this test checks that we can read
those files, and also that we cannot break out of a snapshot
if wide links are not allowed.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=12172

Signed-off-by: Uri Simchoni <uri@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>