tink-tests: add AES-GCM-SIV generator
tink-tests: add AES-GCM-SIV generator

The Rust version included support for AES-GCM-SIV before the upstream
Go version did; this change just adds a single test helper from the
following upstream commit that wasn't already present:
- 1841dd67096a Implemented Go interface for an AES-GCM-SIV Key Manager using the pre-existing subtle implementation.

project-oak/tink-rust,project-oak/tink-rust,project-oak/tink-rust