docker: move testing resources to tests/docker

Relates to https://github.com/shaarli/Shaarli/issues/1153

Signed-off-by: VirtualTam <virtualtam@flibidi.net>

diff --git a/docker/test/alpine36/Dockerfile b/tests/docker/alpine36/Dockerfile
similarity index 100%
rename from docker/test/alpine36/Dockerfile
rename to tests/docker/alpine36/Dockerfile

diff --git a/docker/test/debian8/Dockerfile b/tests/docker/debian8/Dockerfile
similarity index 100%
rename from docker/test/debian8/Dockerfile
rename to tests/docker/debian8/Dockerfile

diff --git a/docker/test/debian9/Dockerfile b/tests/docker/debian9/Dockerfile
similarity index 100%
rename from docker/test/debian9/Dockerfile
rename to tests/docker/debian9/Dockerfile

diff --git a/docker/test/ubuntu16/Dockerfile b/tests/docker/ubuntu16/Dockerfile
similarity index 100%
rename from docker/test/ubuntu16/Dockerfile
rename to tests/docker/ubuntu16/Dockerfile