Bug 1636855 - Enable `editor.truncate_user_pastes` even in Nightly channel r=emilio

Mozilla consider that we should protect even Nightly testers from the behavior
change of bug 1320229.

And I forgot to modify the new mochitest for bug 1635224 which is a regression
of bug1320229.

Differential Revision: https://phabricator.services.mozilla.com/D78841

dom/html/test/forms/test_preserving_metadata_between_reloads.html

@@ -34,7 +34,8 @@
   const str = "aaaaa";
 
   function afterLoad() {
-    SimpleTest.waitForFocus(function () {
+    SimpleTest.waitForFocus(async function () {
+      await SpecialPowers.pushPrefEnv({"set": [["editor.truncate_user_pastes", false]]});
       var iframeDoc = $("test-frame").contentDocument;
       var src = iframeDoc.getElementById("src");
 

modules/libpref/init/StaticPrefList.yaml

@@ -3305,7 +3305,7 @@
 # Whether user pastes should be truncated.
 - name: editor.truncate_user_pastes
   type: bool
-  value: @IS_NOT_EARLY_BETA_OR_EARLIER@
+  value: true
   mirror: always
 
 # How line breakers are treated in single line editor: