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.py

@@ -0,0 +1,8 @@
+import pytest
+
+def test_web_has_no_server_down(docker_compose, nginxproxy):
+    conf = nginxproxy.get_conf().decode('ASCII')
+    r = nginxproxy.get("http://web.nginx-proxy.tld/port")
+    assert r.status_code == 200
+    assert (r.text == "answer from port 81\n") or (r.text == "answer from port 82\n")
+    assert conf.count("server 127.0.0.1 down;") == 0