test: add test_server-down/test_load-balancing

Check that when multiple containers have the same WIRTUAL_HOST and one of
them is unreachable, the resulting `upstream` block has no
`server 127.0.0.1 down;` entry.

test/test_server-down/test_load-balancing.yml

@@ -0,0 +1,30 @@
+web1:
+  image: web
+  expose:
+    - "81"
+  environment:
+    WEB_PORTS: 81
+    VIRTUAL_HOST: web.nginx-proxy.tld
+
+web2:
+  image: web
+  expose:
+    - "82"
+  environment:
+    WEB_PORTS: 83
+    VIRTUAL_HOST: web.nginx-proxy.tld
+
+web3:
+  image: web
+  expose:
+    - "83"
+  environment:
+    WEB_PORTS: 83
+    VIRTUAL_HOST: web.nginx-proxy.tld
+  net: "none"
+
+sut:
+  image: nginxproxy/nginx-proxy:test
+  volumes:
+    - /var/run/docker.sock:/tmp/docker.sock:ro
+    - ../lib/ssl/dhparam.pem:/etc/nginx/dhparam/dhparam.pem:ro