tests: exercise both nginx and caddy proxy backends

Parameterizes tests/common.nix and tests/default.nix on proxyServer
(default "nginx"), giving flake.nix two nixosConfigurations
(test-nginx/test-caddy) and `just build-vm [proxy]` a way to pick
between them for interactive testing.

The automated nixosTest now boots one node per proxy backend and runs
the full assertion suite (factored into check_invoiceninja()) against
each, so a regression in either nginx or caddy's config gets caught.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tests/default.nix

@@ -1,9 +1,10 @@
+{ proxyServer ? "nginx" }:
 { pkgs, modulesPath, ... }:
 
 {
   imports = [
     (modulesPath + "/profiles/qemu-guest.nix")
-    ./common.nix
+    (import ./common.nix { inherit proxyServer; })
   ];
 
   # Required to build pkgs.chromium, which the module pulls in for PDF generation.