Capabilities are serialised in VFS_CAP_REVISION_3 when an image is
built in a user-namespaced daemon, instead of VFS_CAP_REVISION_2.
This adds a test for this, though it's currently wired to fail if
the capabilities are serialised in VFS_CAP_REVISION_2 instead in this
situation, since this is unexpected.
Signed-off-by: Eric Mountain <eric.mountain@datadoghq.com>