feat: add serlo_editor_testing_secret server var #280

Job	Run time
fmt	18s
validate (envs/production)	24s
validate (envs/staging)	22s
validate (modules/api)	17s
validate (modules/cloudflare)	15s
validate (modules/dbdump)	15s
validate (modules/dbsetup)	17s
validate (modules/enmeshed)	15s
validate (modules/gcloud/cluster)	15s
validate (modules/gcloud/gcloud_dbdump_reader)	14s
validate (modules/gcloud/gcloud_dbdump_writer)	13s
validate (modules/gcloud/gcloud_mysql)	22s
validate (modules/gcloud/gcloud_postgres)	18s
validate (modules/hydra)	17s
validate (modules/ingress)	17s
validate (modules/ingress-nginx)	13s
validate (modules/kratos)	14s
validate (modules/mysql)	13s
validate (modules/notification-mail)	16s
validate (modules/postgres)	13s
validate (modules/redis)	13s
validate (modules/rocket-chat)	15s
validate (modules/tls-self-signed-cert)	12s
	6m 8s

Provide feedback