test: lib/uuid: add unit tests for dynamic UUIDs

Add some basic unit tests to validate that the UUID generation behaves
as expected. This matches the implementation in efi_loader for sandbox
and a Qualcomm board and should catch any regressions.

Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>