nixosTests.nextcloud: Add davfs2 example to the VM test

Some people use davfs2 to mount a NextCloud instance, so make sure this
works as well.

nixos/tests/nextcloud/basic.nix

@@ -9,7 +9,22 @@ in {
 
   nodes = {
     # The only thing the client needs to do is download a file.
-    client = { ... }: {};
+    client = { ... }: {
+      services.davfs2.enable = true;
+      system.activationScripts.davfs2-secrets = ''
+        echo "http://nextcloud/remote.php/webdav/ ${adminuser} ${adminpass}" > /tmp/davfs2-secrets
+        chmod 600 /tmp/davfs2-secrets
+      '';
+      fileSystems = pkgs.lib.mkVMOverride {
+        "/mnt/dav" = {
+          device = "http://nextcloud/remote.php/webdav/";
+          fsType = "davfs";
+          options = let
+            davfs2Conf = (pkgs.writeText "davfs2.conf" "secrets /tmp/davfs2-secrets");
+          in [ "conf=${davfs2Conf}" "x-systemd.automount" "noauto"];
+        };
+      };
+    };
 
     nextcloud = { config, pkgs, ... }: {
       networking.firewall.allowedTCPPorts = [ 80 ];
@@ -60,5 +75,6 @@ in {
     client.succeed(
         "${withRcloneEnv} ${diffSharedFile}"
     )
+    assert "hi" in client.succeed("cat /mnt/dav/test-shared-file")
   '';
 })