Dont use gotestsum in hack/test/unit

Dont use gotestsum in hack/test/unit

gotestsum is not yet in Debian, let's just stick to 'go test' for now.

As soon as gotestsum is packaged for Debian (see #940225), we can drop
this patch, and add gotestsum as a build dependency.

This is a partial revert of the docker/cli commits:
- 3bd3996f72ca281cec288dd6e7f4fdaa0e1eeb00
- 277f61415ec99d5fbae75c15013f2fdfb0017af4

Origin: vendor, Debian
Forwarded: not-needed, Debian-specific
Signed-off-by: Arnaud Rebillout <arnaud.rebillout@collabora.com>
Gbp-Pq: Name debian-dont-use-gotestsum-in-cli.patch