nixos/freshrss: fix loading extensions' static content

Before this change, the THIRDPARTY_EXTENSIONS_PATH would end up with a
double-slash in the path, which was breaking FreshRSS's is_valid_path
detection.

nixos/modules/services/web-apps/freshrss.nix

···
     DATA_PATH = cfg.dataDir;
   }
   // lib.optionalAttrs (cfg.extensions != [ ]) {
-    THIRDPARTY_EXTENSIONS_PATH = "${extension-env}/share/freshrss/";
+    THIRDPARTY_EXTENSIONS_PATH = "${extension-env}/share/freshrss";
   };
 in
 {

nixos/tests/freshrss/extensions.nix

···
         enable = true;
         baseUrl = "http://localhost";
         authType = "none";
-        extensions = [ pkgs.freshrss-extensions.youtube ];
+        extensions = [
+          pkgs.freshrss-extensions.youtube
+          pkgs.freshrss-extensions.title-wrap
+        ];
       };
     };
+  extraPythonPackages = p: [
+    p.lxml
+    p.lxml-stubs
+  ];
+  skipTypeCheck = true;
 
   testScript = ''
     machine.wait_for_unit("multi-user.target")
     machine.wait_for_open_port(80)
-    response = machine.succeed("curl -vvv -s http://localhost:80/i/?c=extension")
+    response = machine.succeed("curl -s http://localhost:80/i/?c=extension")
     assert '<span class="ext_name disabled">YouTube Video Feed</span>' in response, "Extension not present in extensions page."
+
+    # enable Title-Wrap extension
+    from lxml import etree
+    tree = etree.HTML(response)
+    csrf = tree.xpath("/html/body/header/nav/form/input/@value")[0]
+    response = machine.succeed(f"curl --fail-with-body -s 'http://localhost:80/i/?c=extension&a=enable&e=Title-Wrap' -d '_csrf={csrf}'")
+    # verify that the Title-Wrap css is accessible.
+    machine.succeed("curl --fail-with-body -s 'http://localhost:80/ext.php?1=&f=xExtension-TitleWrap/static/title_wrap.css'")
   '';
 }