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 commit:
https://github.com/docker/docker-ce/commit/bef34d1

Signed-off-by: Arnaud Rebillout <arnaud.rebillout@collabora.com>
Origin: vendor, Debian
Forwarded: not-needed, Debian-specific

Gbp-Pq: Name debian-dont-use-gotestsum-in-hack-test-unit.patch