tests: hidding docker rm -f error messages
tests: hidding docker rm -f error messages

Since we remove all existing, or non-existing, docker containers we
might see an error message regarding containers not found.
This commit hides non important error messages regarding the
`docker rm -f` command.

Signed-off-by: André Martins <bc9800b9d52a24cce72a73dd528afed53f10e5fc@cilium.io>

tgraf/cilium,michi-covalent/cilium,tgraf/cilium,scanf/cilium,eloycoto/cilium,tklauser/cilium,eloycoto/cilium,eloycoto/cilium,tklauser/cilium,gumpt/cilium,tklauser/cilium,cilium/cilium,cilium-team/cilium,michi-covalent/cilium,tgraf/cilium,tgraf/cilium,scanf/cilium,cilium/cilium,scanf/cilium,gumpt/cilium,cilium/cilium,scanf/cilium,michi-covalent/cilium,gumpt/cilium,gumpt/cilium,michi-covalent/cilium,cilium/cilium,scanf/cilium,eloycoto/cilium,tklauser/cilium,cilium-team/cilium,eloycoto/cilium,scanf/cilium,tgraf/cilium,gumpt/cilium,michi-covalent/cilium,gumpt/cilium,cilium/cilium,tklauser/cilium,eloycoto/cilium,tgraf/cilium,eloycoto/cilium,scanf/cilium,gumpt/cilium