test: skip webui test when not available in build

Add conditional YANG feature 'web-ui' to infix-services.yang, set when
the webui is built.  This feature is what the test now looks for to
determine if it should be skipped or not.

Also, clean up remnant of old test, which was entirely replaced by
Mattias' login/csrf test.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

src/confd/yang/confd.inc

@@ -43,7 +43,7 @@ MODULES=(
 	"infix-firewall-icmp-types@2025-04-26.yang"
 	"infix-meta@2025-12-10.yang"
 	"infix-system@2026-03-09.yang"
-	"infix-services@2026-03-20.yang"
+	"infix-services@2026-06-17.yang"
 	"ieee802-ethernet-interface@2025-09-10.yang"
 	"ieee802-ethernet-phy-type@2025-09-10.yang"
 	"infix-ethernet-interface@2026-05-21.yang"

src/confd/yang/confd/infix-services.yang

@@ -31,6 +31,11 @@ module infix-services {
   contact      "kernelkit@googlegroups.com";
   description  "Infix services, generic.";
 
+  revision 2026-06-17 {
+    description "Add web-ui feature, advertised when the web management
+                 interface (webui) is built into the image.";
+    reference "internal";
+  }
   revision 2026-03-20 {
     description "Add hostname leaf to mdns container for avahi host-name override.
                  Add neighbors container to mdns for mDNS-SD neighbor table.
@@ -71,6 +76,13 @@ module infix-services {
     reference "internal";
   }
 
+  feature web-ui {
+    description "The web management interface (webui) is an optional build-time
+                 feature in Infix.  Advertised when it is built; it gates no
+                 data nodes — the /web service tree (including restconf) is
+                 present regardless, so the feature is a pure capability flag.";
+  }
+
   /*
    * Data nodes
    */

src/confd/yang/web.inc

@@ -0,0 +1,3 @@
+MODULES=(
+	"infix-services -e web-ui"
+)