test: use 0U with gtest to avoid spurious warnings

Signed-off-by: Loic Dachary <loic@dachary.org>
Merge pull request #2053 from dachary/wip-signed-warnings

test: use 0U with gtest to avoid spurious warnings

Reviewed-by: Sahid Orentino Ferdjaoui <sahid.ferdjaoui@cloudwatt.com>