Bug 1826749 - DoH Settings UI tests r=Gijs,settings-reviewers

This adapts previous test cases from the connection UI to the privacy UI.

Additional improvements:
- Set the ROLLOUT_URI_Pref to empty string to avoid DoHConfig reset setting it to undefined.
- Always set network.trr.uri even if it matches the default_provider_uri if it's a user choice.
- If the value of network.trr.uri doesn't match any of the providers, then set custom_uri so
  it shows up in the input text field. This can happen when it's set by a policy.

Differential Revision: https://phabricator.services.mozilla.com/D174876

Author: valenting

browser/components/doh/DoHController.sys.mjs

@@ -249,7 +249,7 @@ export const DoHController = {
         console.error(`Error getting dooh.ohttpURI: ${e.message}`);
       }
 
-      lazy.Preferences.set(ROLLOUT_URI_PREF, uri);
+      lazy.Preferences.set(ROLLOUT_URI_PREF, uri || "");
     }
     this.runHeuristicsThrottled("startup");
     Services.obs.addObserver(this, kLinkStatusChangedTopic);

browser/components/preferences/privacy.js

@@ -583,14 +583,7 @@ var gPrivacyPane = {
         updateURIPref();
       } else {
         customInput.hidden = true;
-        if (
-          menu.value ==
-          Preferences.get("network.trr.default_provider_uri").value
-        ) {
-          Services.prefs.clearUserPref("network.trr.uri");
-        } else {
-          Services.prefs.setStringPref("network.trr.uri", menu.value);
-        }
+        Services.prefs.setStringPref("network.trr.uri", menu.value);
       }
 
       // Update other menu too.
@@ -783,15 +776,20 @@ var gPrivacyPane = {
       document.getElementById("dohWarningBox2").hidden = false;
     }
 
-    if (Services.prefs.prefIsLocked("network.trr.mode")) {
-      document.getElementById("dohCategoryRadioGroup").disabled = true;
-      // The URI is locked by policy. We need to update the custom_uri pref
-      // to make sure the input box contains the correct URL.
+    let uriPref = Services.prefs.getStringPref("network.trr.uri");
+    // If the value isn't one of the providers, we need to update the
+    // custom_uri pref to make sure the input box contains the correct URL.
+    if (uriPref && !this.dnsOverHttpsResolvers.some(e => e.uri == uriPref)) {
       Services.prefs.setStringPref(
         "network.trr.custom_uri",
         Services.prefs.getStringPref("network.trr.uri")
       );
     }
+
+    if (Services.prefs.prefIsLocked("network.trr.mode")) {
+      document.getElementById("dohCategoryRadioGroup").disabled = true;
+      Services.prefs.setStringPref("network.trr.custom_uri", uriPref);
+    }
   },
 
   /**

browser/components/preferences/tests/browser.ini

@@ -95,7 +95,6 @@ skip-if = true
 [browser_primaryPassword.js]
 [browser_privacy_cookieBannerHandling.js]
 [browser_privacy_dnsoverhttps.js]
-skip-if = true # TODO change test to use new DNS over HTTPS UI
 [browser_privacy_firefoxSuggest.js]
 [browser_privacy_passwordGenerationAndAutofill.js]
 [browser_privacy_quickactions.js]