Update TESTING.md to replace testify by gotest.tools
Update TESTING.md to replace testify by gotest.tools

Signed-off-by: Vincent Demeester <87e1f221a672a14a323e57bb65eaea19d3ed3804@sbr.pm>

cpuguy83/cli,thaJeztah/cli,thaJeztah/cli,cpuguy83/cli,docker/cli,docker/cli