Auto merge of #20054 - ferjm:innertext.setter, r=emilio

Implement element.innerText setter

- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #16107
- [X] There are tests for these changes

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tests/wpt/web-platform-tests/innerText/setter.html

@@ -9,6 +9,7 @@
 // tests a not-rendered case.
 
 function setupTest(context, plain) {
+  var container = document.getElementById("container");
   // context is either a string or an element node
   if (typeof context === "string") {
     container.innerHTML = context;