overlay: Add test for follow of lowerdata in data-only layers

overlay: Add test for follow of lowerdata in data-only layers

Add test coverage for following metacopy from lower layer to
data-only lower layers.

Data-only lower layers are configured using the syntax:
lowerdir=<lowerdir1>:<lowerdir2>::<lowerdata1>::<lowerdata2>.

Test that lowerdata files can be followed only by absolute redirect
from lower layer.

Test that with two lowerdata dirs, we can lookup individual lowerdata
files in both, and that a shared file is resolved from the uppermost
lowerdata dir.

There is also test case for lazy-data lookups, where we remove the
lowerdata file and validate that we get metadata from the metacopy
file, but open fails.

Signed-off-by: Alexander Larsson <alexl@redhat.com>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>