Test: Fixup `test_splice_utils`

Actually run 2 different tests...
Support `swift.common.splice`

The `splice` bindings in Swift were reworked in b0f9575be3 to fix some
bugs. This includes an API and semantical change.

This commit updates our driver to support these new bindings, which will
be part of some future Swift version (but most likely won't land in
2.2.1 yet).

See: https://github.com/openstack/swift/commit/b0f9575be3a2ee0ecdcc5148606391e7cc042b5a
Merge pull request #29 from scality/new-splice-api

Support the new Swift `splice` API