[wifi][web_server] Guard AP portal helpers with USE_WIFI_AP and apply review cleanups

tests/components/web_server/common-ap-mode.yaml

@@ -0,0 +1,9 @@
+# AP mode: with a WiFi access point and the interface embedded in the firmware, web_server
+# runs its own captive portal (DNS server, unknown URLs redirect to the page).
+wifi:
+  ap:
+    ssid: "ESPHome-Test"
+    password: "Test1234!"
+
+web_server:
+  local: true

tests/components/web_server/test-ap-mode.esp32-idf.yaml

@@ -1,9 +1,2 @@
-# AP mode: with a WiFi access point and the interface embedded in the firmware, web_server
-# runs its own captive portal (DNS server, unknown URLs redirect to the page).
-wifi:
-  ap:
-    ssid: "ESPHome-Test"
-    password: "Test1234!"
-
-web_server:
-  local: true
+packages:
+  web_server: !include common-ap-mode.yaml

tests/components/web_server/test-ap-mode.esp8266-ard.yaml

@@ -1,9 +1,2 @@
-# AP mode: with a WiFi access point and the interface embedded in the firmware, web_server
-# runs its own captive portal (DNS server, unknown URLs redirect to the page).
-wifi:
-  ap:
-    ssid: "ESPHome-Test"
-    password: "Test1234!"
-
-web_server:
-  local: true
+packages:
+  web_server: !include common-ap-mode.yaml