Bug 1516340 - Properly clean up prefs in browser_contentblocking.js. r=ewright

Differential Revision: https://phabricator.services.mozilla.com/D16030

--HG--
extra : moz-landing-system : lando

browser/components/preferences/in-content/tests/browser_contentblocking.js

@@ -252,6 +252,10 @@ add_task(async function testContentBlockingCustomCategory() {
   await TestUtils.waitForCondition(() => Services.prefs.prefHasUserValue(NCB_PREF));
   is(Services.prefs.getStringPref(CAT_PREF), "custom", `${CAT_PREF} has been set to custom`);
 
+  for (let pref of prefs) {
+    SpecialPowers.clearUserPref(pref);
+  }
+
   gBrowser.removeCurrentTab();
 });