serde_keyvalue: add a few tests
serde_keyvalue: add a few tests

Add tests for a unit type and an untagged enum.

TEST=cargo test -p serde_keyvalue

Change-Id: Ifb840e3270624ef0e54d69b7eba2e631ffcac4e5
Reviewed-on: https://chromium-review.googlesource.com/c/crosvm/crosvm/+/3906752
Reviewed-by: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>
Commit-Queue: Alexandre Courbot <301cd6a3ea50a894b4b51e385c5eb12b9a4ffc0c@chromium.org>

google/crosvm,google/crosvm,google/crosvm,google/crosvm