nixos/tests/common/acme: enable Pebble strict mode

This lets us get early warning about any bugs or backwards-compatibility
hazards in lego.

Pebble will default to this in the future, but doesn't currently;
see https://github.com/letsencrypt/pebble/blob/v2.3.0/README.md#strict-mode.

nixos/tests/common/acme/server/default.nix

@@ -76,6 +76,7 @@ let
     httpPort = 80;
     tlsPort = 443;
     ocspResponderURL = "http://0.0.0.0:4002";
+    strict = true;
   };
 
   pebbleConfFile = pkgs.writeText "pebble.conf" (builtins.toJSON pebbleConf);