chore(mirror): adds drwetter/testssl.sh

Part of https://gitlab.x-ion.de/paas/managed-k8s/-/merge_requests/745 so we always use our Mirror instead of Dockerhub

Why

testssl.sh is the only tool that can give us information about ciphersuites for all TLS versions including tls1.3 reliably, hence this image is needed and used in https://gitlab.x-ion.de/paas/managed-k8s/-/merge_requests/745 to test the ciphersuites of our test-tls-ingress in our e2e tests

Part of PAAS-503